[600MRG] modified MFJ Antenna analyzer failure

[Michael Mussler]

John,
I have been using the Hermes software defined radio with a home-brew coupler to do vector network analyzer measurements on my 600m system. The Hermes covers 100 kHz to 54 MHz and includes a half watt output transmitter.

Here's a link to the TAPR website that has all of the information on it:

http://openhpsdr.org/wiki/index.php?title=HERMES

Public domain software runs the SDR in the network analyzer mode:

http://openhpsdr.org/wiki/index.php?title=VNA

There is a catch to the Hermes approach- Hermes production boards were a limited run and I don't think the loaded board is available any longer. The circuit board and a bill of materials is available and I believe a number of people are ordering the parts and loading their own Hermes boards. It is surface mount technology so that may be an issue as well. The VNA technique should be possible with any assembled SDR that offers both a transmit and receive function. 

I ran some measurements on my antenna matching network and I was able to set up the L-Network so that the VSWR 2:1 points were at the upper and lower band edge frequencies. This has been a good match for the tube finals I use in my system. Weather related impedance changes don't de-tune it enough to cause any problems when I am operating mid band where the best match occurs. I monitor the RF current at both ends of the feed line. There is a detector based ammeter at the transmitter output and a thermo couple based ammeter at the antenna connection to the L-Network. I have good history on the currents at both ends so I can tell if there is a problem in real time. Another approach people are using is to place a Bird Thru Line wattmeter in line. The HF slug has less sensitivity at MF but the decrease is the same for forward and reverse measurements. Since power level is not a factor as long as you can get a reasonable measurement level, VSWR measurements are valid. 

Yesterday I pressed the Hermes VNA into service to check impedance of the short whip antennas I recently set up for receive use. Results help with matching networks  needed to make them work over the frequency range I'm interested in (LF through 30m). I've used the Hermes to track down noise sources and to modify the PA and exciter stages for operation in the proposed amateur allocation. Best of all it can be used as a receiver and exciter on 600m the proposed LF allocation. 

Let me know if I can you provide more information on Hermes or applications.

73
Mike
WD2XSH/12
AI8Z