21cm Line Downconverter Testing

This project is the heterodyning downconverter I have designed and assembled for radio astronomy purposes. It will be used to convert 1420Mhz Hydrogen Line reception from my 10' dish and feedhorn / LNA assembly down to a more manageable frequency of 100Mhz or so. It consists of a mini-circuits mixer along with a Z-Comm VCO for the LO. This particular VCO has a frequency range of 850Mhz to 1600Mhz, ideally covering the 1420Mhz band. A few mini-circuits low noise mmics are used for amplification along with a mini-circuits low pass filter to filter out the original LO and source frequencies.

Last week after returning from vacation I received my downconverter boards and started assembly. Here is the final assembled downconverter ready for testing:



This downconverter as I began testing has excellent performance. I need to do some further testing and measuring to calculate its noise and gain, but I have been very pleased so far. Because of the flexibility of the VCO, I have had some fun during testing downconverting and also upconverting various frequencies around, the following video show upconversion of FM bands to 900Mhz:

Verifying LMR-200 Coax Cable Loss

For years I have used RG-58 cable for my antenna systems, it is inexpensive, easy to work with, and for short runs it works just fine. Last year I started heavily receiving NOAA weather APT imagery using the same RG-58 cable. During this time, pass after satellite pass I noticed my images were not as pristine as they should be. The cable length I was using was just over 100' in length and looking at the loss of my RG-58 it's no surprise that I had such poor images. As a test I replaced all of the cable with a used length on Andrew LMR-400 cable which resulted in an instant improvement.

Now this past week I finally mounted a wide-band scanner antenna on my home and needed a 60' run of cable to reach my RF bench in the basement. Instead of going to my typical spool of RG-58, I wanted something better. LMR-400 was my first choice but is significantly more expensive than my free RG-58. Because of this and the fact that a smaller diameter cable would be better hidden on the outside of the house I chose LMR-200. I purchased 60' of Times Microwave LMR-200 (arguably the best cable you can buy).


With an attenuation of 9.9dB @ 900Mhz, it was significantly better than RG-58. This cable combined with quality Amphenol SMA connectors I was curious to see if the cable would measure up to it's spec sheet. Times Microwave rates it's LMR-200 at 9.9dBm of attenuation at 900Mhz over 100'. At my 60 feet I should only be seeing 5.94dBm of loss which is what I am looking to verify.

The test setup:


An HP 8656B signal generator would be the test source while the spectrum analyzer in my HP 8922H would be used to verify the attenuation of the cable. As a baseline test, I connected a small 1' high quality 50 ohm SMA cable with Amphenol connectors in between the signal generator and spectrum analyzer. Using a -50dBm baseline, the measured signal through the cable was -50.52dBm at 100Mhz. At 900Mhz, it was -51.65dBm, a good start. Now to place the 60' of LMR-200 inline. Unfortunately I did not have a male N to female SMA adapter so I could not test the cable directly inline. To make the test work I placed a mini-circuits ZFSC-2-5-S splitter inline of the cable.


According the the mini-circuits datasheet, at 100Mhz this splitter has a loss on each output of 3.25dBm. At ~900Mhz it has a loss of 3.57dBm, both of which will be compensated for in the results. So testing the 60' of LMR-200 through the splitter at 900Mhz yielded -59.95dBm:


Taking that result and subtracting the loss of the mini-circuits splitter you end up with -56.38dBm, and the difference of that from the original baseline of -50.52dBm at 900Mhz is -5.86dBm of loss at 900Mhz, which is under the spec sheet of -5.94dBm by .08dBm, a very good result indeed. I love it when numbers work out. As a final test I ran a sweep test on the 60' cable using an HP 8754A Vector Network Analyzer. As expected, the loss from 10Mhz to 1300Mhz exactly matched the expected attenuation in the datasheet:



I will be running this cable to my antenna tomorrow. :)

Quadrifilar Helix Antenna for NOAA APT Reception

Since most of my previous attempts of NOAA APT live weather imagery reception have resulted in rather poor quality images, I finally decided to give it a real try this summer. I started with purchasing a serious receiver, the icom IC-R7000. With continuous coverage of 25Mhz all the way to 2Ghz, it is probably one of the best scanner / receivers I have ever used. It has the Japan built quality of electronics that you just don't find anymore and the feel of a very solid piece of electronics. Along with this I purchased a registered copy of WXtoImg, which is definitely the best APT decoding software available. I have used several of the free APT decoding applications, and while they work... they just don't have the image clarity that WXtoImg offers. WxtoImg also can control tuning of my icom IC-R7000 via rs-232 which is very nice as well.

The final missing piece I need is a good antenna. Here is an example of an APT capture using a dipole antenna with ground plane tuned to 137 Mhz:


The image is excellent just above my location ( indicated by the yellow + sign ) but image quality decreases as noise increases quickly outside of my location. What is needed is an antenna with a much better gain from a high speed moving satellite that doesn't have signal fade caused by the orientation of the propagated wavefront. While many people have had good results using basic dipole whip antennas, discone antennas, turnstile antennas, and more exotic Lindenblads, nothing seems to perform as well as the Quadrifilar Helix Antenna (QHA) .

Looking at the QHA, it is initially somewhat hard to understand it's design. It is a pair of circularly polarized, half turn, half wavelength helical antennas designed for reception of low-earth polar orbiting satellites. Reading into the documentation for the actual NOAA polar orbiting satellites, they actually use this exact same antenna design for APT transmission on the satellites themselves. After some research, I came across an excellent calculator for designing the antenna. I recently built this antenna using measurements from the calculator and ended up with this:


It's not perfect, but it is designed to specifications. I used thick 4mm copper grounding wire instead of small copper pipe that other designers of these antennas use mostly for ease of assembly. Also in the picture is the RF choke balun, which converts the balanced signal from the antenna to an unbalanced coaxial cable. It is simply four turns of RG-8 around the antenna mast as close to the feed point as possible. For extra gain, I'm using a mini-circuits ZFL-1000LN low noise amplifier between the antenna and receiver. Overall antenna parts cost was about $30 and assembly time took about two hours. I plan on testing the antenna this weekend and I'm looking forward to some good satellite imagery.