Common Mode Choke for VLF

Adding a common mode choke to the Ground Probe Antenna. I don't think it will help with the noise on 474 KHz as I don't think it is coming out of my house.  Won't hurt to add it. The purpose is to keep noise signals that propagate along the transmission line from getting to the antenna. I wound this using the same core I used for the matching transformer on the Ground Probe Antenna.  It was wound with insulated #18 wire I happened to have.  Around 23 turns, two conductors twisted together. 

I use a drill to twist the wires before winding the core.  Clamp one end of two wires into a vise.  Clamp the other end into the drill chuck and let her wind away. 

Differential mode loss is less than 1 DB.  Common Mode loss was taken with my MDO3000 scope using the function generator to generate flat broadband noise from a few Hertz to past 2 MHz.  This image stops at 2 MHz.

Common Mode Response

The spikes are local AM broadcast transmitters.  The input noise level to the choke was -59 DBm. A table of the data is listed below

Frequency KHz          Loss (DB)

25                                  -13
50                                  -20
100                                -27
250                                -33
350                                -37
450                                -39
500                                -40
700                                -42
1000                              -42
1200                              -41
1500                              -39
2000                              -37

 This will be installed in a junction box that is near the feed point of the antenna. It is about 5 feet away from the connection point. If there are any noise signals on the coax line this should reduce them. 

630 Meter Interference

I had a chance tonight to examine the interference that is blocking reception of the 630 meter band in the evening.  Last night the interference stopped abruptly around 11:15pm local time.  As I suspected the noise returned tonight.  I was out when it started but it was in full force when I returned at 7:30pm.  This gave me a chance to examine the noise in more detail.  First was to look at the audio coming from the receiver.  What this shows is a series of pulses space approximately 8.2 ms apart.  The inverse of this is the frequency which is around 120 hz.  That is why I hear the low frequency buzz in this interference.  I have observed a similar pattern with arching of high voltage distribution hardware.  In that case the noise was very broad band.  Covering from HF into UHF.  In this case the noise is limited to a much narrower bandwidth.  Considering the bandwidth and the way it shuts off I am sure this is a device and not power line hardware related. 

 

Looking at the FFT signature of this same audio sample I see a square pulse that starts around 115 hz and ends around  3.488 KHz.  I think that is why, on Spectrum Lab, I see flat audio response out of the receiver for its full 3100 Hz bandwidth.  The FFT image is shown below:

Looking at a spectrum analyzer from 410 KHz to 500 KHz I see the following.  This essentially wipes out the 630 meter band. 

It will be interesting to find out what this is.  I suspect this is a television or something in the room with a television that household turns on in the evening and turns it off when they go to bed.  So far I have not heard this noise during the day (Monday - Friday). 

To track this down I plan on taking a ferrite rod and build a loop stick antenna. Those have a sharp null off of the end of the rod.  Walking around the neighborhood I hope to be able to use that null to at least locate the house where it is coming from.   Of course I need to make sure it is not something within my own home.  That would be an easy fix if that was the case.  Since this shut down last night around 11:15pm I can not think of anything in the house that would have done that last night. 

Ground Probe Antenna

Ground probe antenna which some call them Earth Probe Antenna.  It consists of a long wire terminated with ground rods at each end.  Looks similar to a Beverage antenna which is a very long wire terminated at the end.  The wire being only a few feet off of the ground.  These are low noise antennas directional off the end.  Best used for DX on 160 meters as their directivity and pickup of local noise is much less than an Inverted-L for example. 

The Ground probe antenna is used for reception of ELF and VLF signals. This post will describe the one I put out on my property in the suburbs.  My back yard is around 175 feet wide and I was able to run a wire out 158 feet (41 meters).  For now the wire is just laying on the ground.  I drove 8 foot ground rods at each end.  I connect to the antenna approximately in the middle.  I am not sure how to feed this antenna so I did so in the middle as that was a convenient location.  Some on suggested the impedance magnitude was around 1000 ohms.  With further measurements I think it is more like 500 ohms. 

I have in my junk box a ferrite core of unknown properties.  I measured the AL value at 6490 mH / 1000 turns.  The material appears to be "P" or "F" ferrite used in switching power supplies running under 1.5 MHz. I am feeding the antenna with RG-6 cable.  I used 50 turns on the antenna and 14 turns connected to the RG-6.  I am going to change that when I get a chance assuming a 500 ohm Zmag rather than 1000 ohms.

I drove the ground rods below grade with a 12" long section of 4" PVC at the top with a cap.  The cap is flush with the ground so I can run the mower over the top.  I can still get to the ground rod connection by removing the cap.  I did the same for the coax connection in the center.  The transformer sits inside of the 4" PVC. 

 

First tests

It was 20:00 EST on February 3^rd when I connected the HP3586C up to the antenna.   Wide band noise was -8 DBm with the broadcast stations.  Cutting out the big three (WHIO, WING, WONE) with my trap filter the broadband noise was -21 DBm.  Noise with 3100 hz bandwidth on 630 meters sits about -86 DBm, except…… well I will get into that shortly.

Daytime levels for WHIO are -13 DBm and -10DBm for WING.  They are +10 DBm on the 160m inverted-L.

First to test was the broadcast band.  The antenna did very well from 550 to 1500 KHz.  Picking up many stations at night.  Better reception on the typical ones such as KDKA, WBZ,  WHAS, etc.  I did not attempt to dig down for the weaker stations.  I tried FT8 on 160 meters and that was a failure.  Only decoded three stations.  

Next test was WSPR on 630 meters.  A while back I ran this test on the K9AY loop before I added the 160 meter high pass filter to cut out everything below 1.7 MHz.  That did very well.  So how would this antenna compare?  Well…. I ran into a brick wall.  Or I should say, a noise wall.  Before I said the background level was -86 DBm on the 86C.  That was after midnight.  At 8pm the level was more like -66 DBm.  It started around 400 KHz and ended below 550 KHz.  I tried to determine a signature of the noise but could not find any frequency peaks or repeating pattern. It has a low frequency buzz similar to 120 Hz.  I could not hear anything through that junk.  To my surprise it abruptly ended around 23:15.  Someone shut something off.  It is not present during the day.  I am leaving the receiver on to try to determine the hours of operation of the QRM.  When it is off I hear 630 meter WSPR very well.  Image of map attached.  On this map notice a K9 stations in Hawaii.  At first I thought that was bogus but looking at QRZ he does live there.  There was another “W4” stations in the southern pacific.  If that guy is really there he has probably drown by now.  His QTH is listed in Tennessee.  As far as I know Tennessee has not annexed that part of the Pacific.  I assume he has his grid square setting messed up.  There are three stations on this map that did not appear on K9AY test.  A station in western Canada, the K9 in Hawaii, and a station in California. 

Next test was reception of Non-Directional Beacons (NDB) for aircraft.  List of the stations attached.  A couple of stations to note.  DDP is a 1KW beacon in San Juan, PR.  I am not surprised at that one.  The one I question is BBD in Brady, Texas.  According to the database that is MH class station. Those have a maximum power of 50 watts.  The database shows it as 25 watts which is typical of the class “compass locator”.  It was very weak but it was there and the frequency agrees with the database.  As you can see from my list all the other 25 watt “compass locators” were more local.  Last night I picked up three new ones.  One being QT which is a 1KW beacon on 332 KHz out of Thunder Bay, Ontario.  So far I have logged 29 NDBs.  I also received numerous of the Differential GPS (DGPS) stations.  I found a list of them all but did not make any attempt to log or identify those.

Of course the strong NAA out of Cutler, Mane, on 24 KHz was present.  I also could detect two other near 24 KHz. 

I have not heard anything on the Amateur 137 KHz allocation. 

Last night I could hear lightning static crashes.  Looking on a live lightning map the nearest thunderstorms were in the Atlantic near Bermuda.  The map indicated it was running a 4 second delay.  When I heard strong crashes four seconds later I could see a dot light up on the map.   So I got lightning DX into the middle of the Atlantic.  

As for that noise in the 630 meter band I plan on winding a ferrite loop stick antenna and see if I can track it down.  I fear it is neighbor's TV set.  At least they shut it off when they go to bed.   Maybe I will have more information in the future to post in this blog.  

Ground Rod below grade in 12" PVC pipe so I can get to the connections

Cap covers rod and transformer connections

Transformer for feed point to RG-6 coax

630 Meter WSPR station I heard the night of February 4, 2019

Antenna Voltages Near Broadcast Stations - 160 Meters

Wow!  It has been a long time since I posted to the blog.  I am still experimenting with various topics in Amateur Radio.  Just have not taken the time to document them here. I am now retired so will have more time to play with electronics and radio.

Recently I have been improving my 160 meter antenna system. I made a few changes to the Inverted-L antenna by spacing it farther off of the tower and getting the end wire out of the tree.  I still need to get the end wire higher up in the tree.  The wire is around 134 feet long.  80 feet go up the tower and the remainder horizontally toward a tree to the south.  The end of that wire is only about 25 feet above the ground. It should be more horizontal.  That tree is much taller but I can't get up in it any higher without a bucket truck or a man-lift.

I also added eleven radials fanned out over the back yard.  The radials are 67 feet long.  There is a reason for that which I should document in a future post.

The 160 meter band is just above the AM broadcast band.  I live with in 2 miles of the big three Dayton, Ohio AM radio stations:  WING, WHIO, and WONE.  These are 5000 watt stations running omni directional antenna during the day and directional at night.  The inverted-L picks up a log of RF during the day when the stations are running an omni pattern.  At night the directional pattern appears to be away from me as the signal level drops for all three stations.

For example, looking at WHIO (1290 KHz) using a frequency selective volt meter I see +16 DBm during the day on the inverted-L. At night it drops below 0 DBm.

Today I hooked up four 1N4148 signal diodes in a full wave bridge configuration.  I hooked that to the inverted-L antenna.  I get 38 volts open circuit from the diode bridge.  Short circuit current is 8 ma.  I placed an LED across the diode bridge output.  It lights the LED right up!

 

During the day I also have intermod problems from the high power stations combining together.  One 3rd order mix fall right on the 160 meter FT8 frequency.  Of course, 160 meters is dead during the day and the intermod mix disappears once the stations go to their directional pattern at night. This has not been a problem but it would be nice to eliminate the issue.  

At first I tried putting high-pass filters on the receiver. Thinking the intermod was from overloaded receiver front end.  That did not work so that tells me the intermod mix is coming from something external.   That could be anything.  Mixing in one of the transmitters, mixing from a rust bolt somewhere.   I am going to try to find it.  I am thinking a ferrite rod antenna built to resonate on 1.840 KHz might work.  Those rod antennas have a sharp null off of the end.  If it on my property maybe I can find it.   Snowball's chance in hell, but I will give it a try.

Looking At My Antennas With The HP8753B

Using the HP 8753B Vector Network Analyzer I took a look at what my Classic 33 Yagi and 3-band fan dipole looks like from the shack. The Classic 33 is my original that dad got used back in the 70's It was refurbished once and installed at this QTH in 1989. The antenna has not been touched since. The feedline is Belden 9913 which was installed in 1989. Getting concerned that this air core coax may have water in it after all these years. For today's test the outside conditions are dry and have been for a couple of days. The Fan Dipole I put up last year. It consits of three dipoles feed from a common point. The dipoles are for 40, 30, and 17 meters. Dipole spacing and length of the elements are from a military document (Army I think) I found online. This document was written back in the 50's or 60's if I recall. It is installed as a sloper off of the tower favoring the west. I used the old 9913 coax that feed the 40-meter inverted V that this antenna replaced.

Looking at the Classic 33 first

I don't have a way to download the screen shot of the VNA so I used my Canon 30D camera to take a picture of the screen. I see I need to work on the focus. I used a tripod in front of the VNA but it was easily moved out of postion. If I am going to continue to use the camera I need to build a fixture that attaches to cart that holds the VNA.

The first screen shot is a Smith Chart view on 20 meters.

I was playing with the buttons on the VNA so now on to the Return Loss and SWR plots.

The SWR on the Classic 33 is not very broad on 20 meters. The yagi elements were installed favoring the voice part of the band. So the low end of the CW band suffers. SWR is 3.2:1 at 14.0 MHz, 1.16:1 at 14.2 MHz, and 1.6:1 at 14.3 MHz

15 Meters is better with the SWR ranging from 1.75:1 at 21.0 MHz to 2.0:1 at 21.450. Best SWR at 1.43:1 is at 21.160 MHz

10 meters is a wide band. The Classic 33 was intended to be used in the lower part of the band below 29 MHz. Installed for the SSB part of the band SWR ranges from 1.87:1 at 28.0, 1.17:1 at 28.3 MHz, 1.57:1 at 28.5 MHz, and 2.28:1 at 29.0 MHz. This image shows the return loss as well as the SWR in ().

The results of the tests look as I remember them when the antenna was installed at this QTH. After I made these tests I disconnected the yagi and placed a short on the end of the feedline. Using a return loss measurement and calculating the length of the coax line I came up with the following:

Length of line = 156 Feet

Loss measured at 50 MHz = 2.4 DB

9913 has a loss at 50 MHz of 0.9 DB/100 Foot according to one datasheet I found online. At 156 feet the loss per this specification is 2.8 DB. So this loss is within spec give or take my measurement error.

My notes from the installation back in 1989 indicates the disance from my connector panel in the shack to the connector at top of the tower is 136 feet. All the measurements I have made using the VNA or an MFJ analyzer show the length of my line to be 156 feet. Not sure why I have a discrepancy of 20 feet. I plan on changing out this feedline so I will measure what I pull out and see if my VNA or my notes are correct.

Now the Fan Dipole

Looking at the Fan Dipole return loss from 5 to 55 MHz you can see the three bands. Also a good return loss is noted around the 6 meter band at 49.5 MHz just a little shy of our allocation.

Looking at 40 meters the SWR ranges from 1.34:1 at 7.0 MHz to 1.8:1 at the top of the band, 7.3 MHz.

Moving to the 30 meter band. SWR is highest at the lower end of the band. 1.46:1 at 10.1 dropping to 1.20:1 at 10.180

On 17 meters the lowest SWR of 1.03:1 is below the band at 17.740 MHz. For the lower band edge of 18.068 the SWR is 1.7:1 rising to 1.8:1 at the top of the band. If I get ambitious I may try to shorten the 17 meter dipole a bit to pull the lower SWR inside of the band.

GE Mastr II Repeater

I have not done much in the ham shack for a while. Have many project to do but it seems like the time is spent in other areas. That is life I guess. Well I have a new projet that I have to get finished.

I am converting a GE Mastr II base station from commercial service to amateur radio repeater service. The original system was used as a base station up around 155 MHz. The GE is specified from 138 to 174 MHz but there were some component changes in models that operated below 150 MHz. Looking at web sites on converting these systems only the receiver has a few capacitors that need changed for optimum performance. The first step is to confirm the unit is working as it came out of service. Cleaning up the system I found there had been a mouse nest in the power supply. Had to clean that out. Apparently no damage caused by the unwelcome guests. Now on to testing the system. Testing the transmitter I found the output power was only 1.5 watts. That is a little off from the specified 100 watt output. There is a known issue where a strap that was placed between two PC boards to couple the output power from the finals to a filter develops a hairline fracture. The symptoms pointed to that as a possible problem. Turns out that was not the case. Since this was used in non-repeater service there is an antenna change over relay on the output. The relay had gone bad. Since I am using this for repeater service I don't need the relay so I pulled it off of the board and jumpered across it with a small length of RG-316 coax. Problem solved. I can develop the full 100 watt output now. For testing purposes I set the power output to 50 watts while I am doing the conversion. Next step will be check the receiver.

Catalog Filters

I was working on a project for the radio club and needed a couple of filters. One for 571 Mhz and the other for 735 Mhz. Previously I made a stripline filter and I have done copper pipe filters. These are rather large and I wanted something quick. Looking in the Digikey catalog I found filters by Toko. If you are buying a million of these filters I am sure Toko would make them exactly to your spec. For ham use in single quantity we have to take what is stocked in the catalog. Of course the catalog selection was made without consulting us hams to what we might need. The chance of finding something in stock value that is useful approches Epsilon. Much to my surprise I found a helical filter for 734 MHz with a 10 MHz bandwidth. Ok, lucked out on that one. For 571 the closest I could get was 550 MHz. Could I tune it up to 571? This little two section filter has tuning screws at the top of the cans. The screws were near the top so it looked like I was already at the top of the frequncy limit. As it turned out I was able to tune up to 571 MHz. For a test I ran the screws down to see how low I could get and still have the same overall filter response. The stock specification was Fo = 550 MHz, 10 MHz bandwidth. I don't recall the published insertion loss but I was looking at about 5DB. That was measured at the dip of the ripple which was approximatly 2 DB. My test jig was not ideal for grounding the cans, terminating the filter, and making connection to the RG188 teflon cable so I am sure I introduced some error in the response. The results:

Minimum Fo = 494.1 MHz
Maximum Fo = 586.0 Mhz

Other specs remained about the same but the insertion loss and bandwidth was increasing as I approched the upper limit.

For this filter I can pull about 8% from center but the catalog frequency was already near the top of the range. This would be unknown to the purchaser until after they examined the product.

If you need a filter check the catalog offerings from Digikey and Mouser. You may luck out and find something that might tune to the desired frequency but I would not try to go beyond 8 to 10 percent.

For reference this part was Toko type 7HW, Part number # 252HXPK-2736F (Digikey #TK3307-ND)

Followup on Cheap Cables

This is a follow up of the cable problem of the previous post.

From the photograph you can see there is minimal copper braid but an inner foil around the center dielectric. Looking into the cut-a-way connector body I found the crimp was applied metal against cable jacket. There were strands of the copper braid sandwiched between the metal connector body and the cable jacket. That was the extent of the ground connection. The foil did not touch the connector body as the inner chamber of the connector widened out past the cable jacket. I suspect the shield connection had been compromised. The bad connection could have been caused a number of factors such as the copper braid being pushed into the jacket, corrosion of the copper wires, or breakage of the wires.



Moral of the story is if you obtain cheap cables they may work fine but keep in mind they may have limited life. Don't be afraid to chuck them at first sign of trouble. I still prefer my homemade cables made from LMR-240 Ultraflex and the correct connectors for that cable type.

Cheap Cables

I picked up a box of 3-foot and 6-foot BNC cables. The cables are marked as RG58. Overall construction of the cables look good as they have molded ends on the BNC connectors. Unfortunately good looks is where it ends. Electrically the cables suck. A good example raised its ugly head today. I was checking a function generator and connected it to my HP 5335 frequency counter. Ran through a few frequencies to make sure the calibration of the generator was reasonable. Higher frequencies no problem. When I went to 1 KHz the counter would not lock onto the signal. I connected the generator to my oscilloscope and observed a nice 1 KHz sine wave. Playing around with the counter I engaged the filter switch and got a correct measurement of 1 KHz. Looks like I am getting noise in the input on lower frequencies that prevents the counter from locking onto the desired signal. I placed a few clamp-on ferrite chokes on the cable and that also took care of the problem. I then replaced the cheap BNC cable with one I made from LMR240 ultra flex. Problem went away without any filtering.

Moral of the story. Buy good cables they will save you headaches in the long run.

ABS problems on the van

I am having ABS issues on my van. The ABS brake system kicks in at very low speeds on dry pavement. There is a service bulletin about rust on the mount point of the sensor causing it to pull away from the hub so the signal is erratic. That would be a simple fix as the sensor is $200. I can not find a cheap overseas replacement part even on Ebay. My scan tools shows a couple of generic ABS chassis codes:

C0281 (chassis code - ABS) Brake Switch Circuit
C0223 (chassis code - ABS) Rt Front Speed Signal Erratic

I suspected the right front wheel so this confirms it. The test procedures specify resistance and voltage readings for the sensor. Getting the voltage reading is going to be a trick as I can not spin the front wheel as I have all wheel drive. I will have to disable the ABS then rig up a cable to attach to the harness so I can roll the van down the driveway to measure the AC pulse train. The voltage is very low (approx 100 millivolts) so noise is going to be a problem even using the oscilloscope. The service bulletin indicates the signal should be at least 350 mV but this document says greater than 100 mV. So much for consistent documentation.

The procedure also measures the cable from the sensor to the electronic brake control module (ECBM). It states that faulty wire connections is a leading cause. The darn EBCM is attached to the frame under the van. That is the dirtiest place you could put the thing.

I am a little confused by the C0281 code for the brake switch circuit. The description indicates that this code may set if the there is an ABS fault of at least 500 milliseconds. That would have set by the C0223 code but then in the next line it says the code only sets if there is an open or short in the brake switch circuit wiring. For now I will assume it is setting because of the sensor signal. Fix that and then see if C0281 comes back.

The last item in the correction chart is to replace the ECBM. Unfortunately you have to have a scan tool that communicates to the device. Consumer scan tools only read diagnostic codes. You need on of the expensive units to talk to the devices. You have to set the tire diameter for the vehicle in the ECBM. Crap! If that is bad it is back to the dealer to replace that darn thing. I bet they get a lot of $$$ for that module.

Over the weekend I hope to get a chance to investigate the sensor/wiring.

UPS Batteries

Test complted on the batteries in the UPS.  There are four PowerSonic PS-1272 7.2Ah batteries in the UPS.  Three of the pass as shown in this graph:

 

 

One battery would not carry any load.  That one is the problem. 

 

Results of the test in text format:

 

 

 

UPS Failure!

I have a 1400 watt rack mounted UPS that powers my computers. The computers and UPS are in a rack. Yesterday I discovered all my computers rebooted in the early morning. The UPS appeared to be running fine by way of the indicator lights on the front. I gave it the big test by pulling the power cord from the wall. The entire rack went dark. The UPS was running in bypass apparently. I brought the UPS back on line and now I had a red malfunction light on the front panel. This is a refurbished UPS which is one of two that I have. They are about a year old. I swapped the failed unit for the other one which is only running one PC downstairs. I am now in process of checking the batteries. I have noticed the battery charge status leds have been flashing. I have a technical bulletin about that indication. The document talks about a calibration process that may remove that warning. Before I do that I need to test the batteries to make sure they are still in spec. There are four batteries in the UPS. They are 12 volt Power Sonic 7.2AH batteries. They are connected in series and parallel providing 24 volts at 14 Ah. I have about 650 watts on this UPS so that only gives me about 10 to 15 minutes run time before they crap out.


I test the batteries using a computerized test fixture. This is a device by West Mountain Radio. Plugs into the USB port on the PC and by running the provided software you can do a load test and see if the battery meets specification. You can see the resulting chart after the test on the first battery. You set the load current as indicated by the specification. Most batteries of this type are tested over a 10 or 20 hour rate. Power Sonic lists the spec for both 10 and 20 hours so I used the 10 hour rate. For a 7.2 AH battery that is 0.700 Amps. They also specify the "dead" voltage which is typically 10.5 volts for a 12 volt battery at this load current. As you can see from the chart I got 8 Ah out of the battery by the time it hit 10.5 volts. So battery 1  is good. This battery goes on the charger and I connect up the second. At ten hours a test this will take a little time.


Chart after test of first battery



Batteries ready for testing. Battery #1 connected to the test fixture



Batteries as wired up in the UPS unit

New Toy

I picked up an HP 5372A Frequency and Time Interval Analyzer. This box take time interval measurements: time interval, continuous time interval, and +/- time interval. A time interval is a measurement of elapsed time between two electrical pulses. I will use it for testing and comparing oscillators. As I sit here playing with this unit I wonder what kind of unusual time interval analysis has been done using this type of device in industry or the military.


I have completed the functional tests out of the manual. The unit appears to be in good shape and calibration.


 

The SUN's solar activity and climate change

All this crap about man being the cause of global warming never ceases. In the political arena man is to blame and everyone totally discounts the effects of the SUN. What a bunch of moron’s using nature’s normal cycle as an excuse to generate taxes (cap and trade/carbon emission reduction) and force lifestyle changes on the population. Years ago I took a course on the Sun’s effects on radio propagation. This was long before Owl Gore and his CO2 vs temperature chart (which he read backwards by the way). It was quite apparent that the 11 year solar sunspot cycle’s overall trend has the greatest influence on the Earth’s temperature. Much more than what man can do to the planet. We have been at the bottom of the last solar cycle for a much greater period of time then cycles in the past. If the new solar cycle had followed the pattern of previous cycles we would now be a couple of years into a rising sunspot count. This pattern has not occurred in this cycle. We have been in a period of no sunspot activity. We have been in the low period of sunspots for a few years. The comment below speaks of the period of time where NO sunspots have appeared at all. Continued low solar activity will drive the Earth temperatures down. In the current political arena the wording is changing from “global warming” to “climate change”. That way they cover all bases no matter what the temperature does. They can continue to spread lies about climate change to effect new taxes and control of the population.


This is comment was in a DX news letter that follows along with what I found when I took the propagation course.


Out in space the sun remains on a record pace at 51 days without sunspots, and no expectations of any change soon.  We’ll be in the top 3 longest quiet periods by Wednesday evening (tied with 54 days from 1879) and by late next week we’ll head into the 2nd position as we surpass 63 days from 1901. The daily solar observations began in 1849, but we know from other observations between 1400 and 1830 that these
periods of sunspot minimums were associated with a 400-year stretch known as the Little Ice Age. The only problem with observations from that time is that there were no telescopes and satellites with the observational power we have today. In other words, the long periods without sunspots are even more impressive now because we can pick out the smallest sunspot, ones that would have been missed 100 to 200 years ago. We
won’t know if this is true for a long time, but at this point there is no reason to discount the idea that we’re heading into another Little Ice Age, or possible a major ice age. Time will tell…

Testing ERA-5 MMIC for use as a multiplier

An easy way to generate harmonics is to drive an amplifier into compression. MMIC amps are easy to use and the 1 DB compression point is listed in the datasheet for just about all MMIC amplifiers available today. Normally this would be an undesired operating point. If you need a multiplier this a simple way to do it. Output are typically higher using an MMIC as a multiplier than using a simple diode multiplier. The output circuit of the MMIC can be designed to enhance the desired harmonic. That was not done in this test.

ERA-5 gain below 1GHZ = 20 DB
1 DB compression = +17.5 DBm

Input 738 MHz into MMIC at +6 DBm and +2 DBm drives the amp well into compression. No attempt was made to optimize the output for a particular harmonic or to suppress the fundamental. The output capacitor was 12 pf to favor the upper frequencies. Results varied depending on the input frequency. The data below is for 738 MHz. At 571 MHz the 5th harmonic with +2 DBm input was -20 but had a lot of “grass” around the carrier. Lowering the input to +1 DBm dropped the harmonic to -30 DBm and cleaned up the noise (or it dropped below the noise floor of the spectrum analyzer).

It appears the input should not exceed 5 DB above the 1 DB compression level or excessive noise will result.

+6 DBm input is 8.5 DB above the 1DB compression point
+2 DBm input is 4.5 DB above the 1DB compression point

Output 1 is with a +6 DBm input
Output 2 is with a +2 DBm input

Frequency(MHz) or Harmonic/Output1/Output2
738/+20/+18
X2/-2/-6
X3/+3/-12
X4/-4/-30
X5/-20/-30
X6/-42/-34
X7/-38/-46
X8/-30/-42
X9/-40/-50
X10/(below noise)/(below noise)

Oscillators

I assembled the two transponder oscillators yesterday. One is for 82 MHz and the other for 114.2 mHz. They are based on the initial design I did last summer. I thought I had these all figured out. The 82 MHz oscillator just sucked. The harmonic content was very high. Hell, I did not need the multiplier stages I designed. I had a 738 MHz harmonic from the oscillator directly only 30 DB below the fundamental!! As it turned out the little MMIC amp I had at the end of the oscillator was being overdriven. I just removed the MMIC and jumpered the board from the end of the buffer to the SMA connector. I have a good -5 to 0 DBm which was the initial design goal. I also removed the filter components after the buffer. I was getting a spur around 300 MHz. I don't know if it would have gone away by removing the MMIC. I removed the filter parts before removing the MMIC. I suspect circuit board traces in the filter caused the instability. I probably had stray capacitance and/or inductance somewhere in the filter physical layout that screwed it all up.

Other changes I made:
1. Removed the tuned transformer between the oscillator and buffer. Replaced it with a short to the DC line.
2. I moved the input capacitor of the buffer that was on the transformer link to the top of the oscillator emitter resistor.
3. I also reduced the buffer emitter resistor from 220 to 100 ohms. This helped reduce the harmonics a little. I think the buffer was also being overdriven.

I experimented with the tapped capacitors in the oscillator tank circuit. Increased them to 82 and 33 from 68 and 22 pf. No real difference. The oscillator may be slightly more stable, from a start up perspective, with the original caps so I put them back to the way they were.

In the end the 2nd harmonic is 10 db down and the others are 30 db down or more. The 5th harmonic is the last one I see on the spectrum analyzer. The input tuning on the multiplier may make these spurs a moot point. If not I will have to add an extra low pass filter after the oscillator. The final oscillator is much simpler design.

I will incorporate the design changes into the 114.2 MHz oscillator and test it out tomorrow night.

I guess I will have to go back and update the design document on my web page. That will have to wait until after Hamvention. I need to get this project done by Hamvention.

Interdigital Filters - remade

I etched the new boards over the weekend. The frequency of the 738 and 571 filters came out spot on. The pesky "step" is still there especially on the 571 filter. The loss is less on the 571 but that was expected per the simulation using FR4 board stock. The return loss just sucked on both. I ended up adding a surface mount trimmer cap (3 to 12 pf I think) in series with the input on the 571 filter. I also moved the input tap up several millimeters. About as far as I could reach with the body of the trimmer. Adjusting the cap I was able to achieve a return loss of 18 DB. Slightly above the desired frequency I could get 20. I adjusted for best RL on the design frequency. When I get some time I will add the same capacitor to the 738 MHz filter. For now these are done and I will go on to other modules of the project.

Making New Artwork for 738 Interdigital Filter

One final mucking with Ansoft dimensions on the interdigital filter. I started by changing the microstrip impedance to 82 ohms (40 mils on this FR4 board). It was around 40 or 50 before from the initial Ansoft design. That appears to work much better. On the pipe cap filters the ideal impedance of the pipe coax structure was 77 ohms so that must hold in general for the micro strip filters too. The filter response is much better with the best return loss also being the center of the band pass response. The RL looks to be 15 or so. Insertion loss is still high but down under 15 DB now. Out of time for tonight so I will re-do the PC board artwork in Eagle tomorrow night.

Update on Frequency Error in Ansoft Design

I found the source of the 100 MHz frequency error in the earlier post. Ansoft converts the theoretical filter design into a physical layout. This layout is more of a schematic then a circuit board layout. The four microstrips of the filter are modeled in Ansoft as four rectangles. Three sets of these rectangles make up the entire filter. Each strip is broken up into three pieces because of the input and output taps. The taps are represented by a “T” symbol that has no length, just width. To position the tap the total length of the line must be broken up in the model representation. There is some confusion as to what length “p” is in relation to the total length of the line. Also the conversion process did not use the variable “p” but instead inserted the total length of the line. When I made the board I misinterpreted the meaning of each length “p”, i.e. p1, p2 and p3. It was not clear how these added up to make the total length. I ended up with each filter element being about 250 mils too long. This made the filter resonate 100 MHz low. Now that I have figured this out the simulated frequency and measured frequency is within reason give just an uncertain dielectric constant of FR4 at these frequencies.

Measurements on the N4 Interdigital Filter

After messing around with this interdigital filter I never could get a nice smooth response. At best there is “step” in the response as the frequency goes above the center frequency of the pass band. For this application that would not be a problem as I am just removing other harmonics of the main oscillator. Using FR4 board is not the best for UHF and above filters. The loss is just too high. Also the dielectric constant is not very constant. Now that I have this filter built I measured the final microstrip elements.

P (total length of each microstrip, there are four) = 2062.5 mils
P2 (tap for the input and output from ground end) = 203 mils
W (width of each microstrip, they are all the same) = 164 mils
S1 (spacing between element 1 and 2) = 391 mils
S2 (spacing between element 2 and 3) = 172 mils
S3 (spacing between element 3 and 4) = 500 mils

Measuring the actual center of the band pass on the tracking generator indicates 737.68 MHz. Now we can modify the simulation with our actual measured values and keep changing the dielectric constant until the frequency matches the measured. Nominally FR4 is around 4.5. The simulation shows:

For 4.7 Fo = 743.53 MHz
For 4.8 Fo = 736.39 MHz
For 4.9 Fo = 728.57 MHz
For 4.79 Fo = 737.68 MHz

Setting the simulation to 4.79 yields our measured frequency of 737.68. We will use this value for future filters around this frequency. I want to try a hairpin filter next. I have the initial design. Other than the narrow spacing and I assume more critical layout tolerances the pass band does not contain the “step”. Hairpins do respond to the even harmonics of the first pass band. I don’t think that will be a problem the higher harmonics of the multiplier will be reduced by the design of the multiplier.