Hardware for Investigating VHF/UHF Path?

Discussion in 'Ham Radio Discussions' started by KK5JY, Jul 13, 2019.

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  1. KK5JY

    KK5JY Premium Subscriber QRZ Page

    When the weather starts to cool off in a few weeks, I have a path between two different locations that I would like to investigate using 2m. The idea is to figure out what minimum antennas + installation height will get a usable path. I'm planning to do the investigation using FM packet, for simplicity and minimizing cost. Once I have the results, I can extrapolate to other modes, as needed, and if needed.

    For the antenna part, I'm planning to use some telescoping fiberglass, so I can vary the height without too much difficulty. The antennas would be horizontally polarized yagis.

    For the rest of the gear, I'm thinking about a RasPi with a TNC-Pi at one end, running from solar + battery, to give me an endpoint that I can query without having a person there. That end is just open field, so it would need to be something I could put in a box. The radio would just be a Baofeng, or whatever I could get cheap.

    The idea would be that I would regularly interrogate the remote unit, and then see how much the error rate and/or packet loss rate improves or worsens at different installation heights, but averaged over time. That way, I could do most of the experiment from one end of the link.

    Any thoughts on better hardware for a remote + rural digimode endpoint for testing purposes?
  2. KK4NSF

    KK4NSF Ham Member QRZ Page

    where are you? Around here, it's going to be hot as all hell for the next 2 1/2 months!
  3. KD8TUT

    KD8TUT Platinum Subscriber Platinum Subscriber QRZ Page

    The easier way would be to put a radio that has APRS capabilities at both ends, feed the output via serial to a computer or tablet running APRSIS32. You will always know what the band conditions are.
  4. WA9SVD

    WA9SVD Ham Member QRZ Page

    Most V/UHF FM stations use VERTICALLLY polarized antennas. If you use a horizontally polarized antenna, you suffer at least 20-30 dB attenuation of each station.
  5. KK5JY

    KK5JY Premium Subscriber QRZ Page

    My plan is to use horizontally-polarized antennas at both ends of the link.
  6. KK5JY

    KK5JY Premium Subscriber QRZ Page

    That's an interesting idea. Originally, I thought about using one of these at one end:


    ...and just making the test a one-way passive test, where the beacon device sent out a transmission every few minutes. The reason I am leaning towards a full AX.25 station at both ends is so that I can do a more proactive test, where I send data in one direction, and get responses back.

    But perhaps my idea for a bidirectional test isn't as big an improvement over the one-way option as it might seem?
  7. K7JEM

    K7JEM Ham Member QRZ Page

    This is what I would do:

    Set up for a one way test, as you describe here. The path for the most part should be reciprocal. Set up the far station as a simple beacon to send MCW over your FM radio, Baofeng or whatever. Set it to TX callsign, etc, every ten minutes, or whatever you want. Then, at your home end, use a radio that has an RSSI output, such as the Motorola CDM series of mobiles. Calibrate that against a chart to see what the actual signal level is. Have the Baofeng send a PL tone along with the CWID. At the home end, program the mobile to decode that PL. Have your computer log the RSSI every time the PL decodes, this will prevent logging false signals.

    When you are done with your tests, resell the used CDM on e-bay for close to what you paid for it. These radios usually sell for around $100, sometimes less. You would need to find someone with the programming software to program it for you. Here is a link to some of the things the back connector can do:

    KK5JY likes this.
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    Why would the path change if you have two antennas pointing at each other ?
  9. KK5JY

    KK5JY Premium Subscriber QRZ Page

    Hopefully it wouldn't change much, but it is just over 40 statute miles over varying terrain. The part that would change with certainty is the antennas at the ends -- I was planning to vary the height slowly over time to see how that changed the usability of the link. Depending on how that worked out, I might also change the gain of the antennas, as well, to see how small of an antenna gives me reliable results.
  10. KK5JY

    KK5JY Premium Subscriber QRZ Page

    That shouldn't be a problem. I don't have access to this myself, but I know people who do.

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