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  1. #1
    Join Date
    4th December 2017
    Posts
    3

    RDF with the TeleMega: suggestion for a ht

    I'm fairly new to HPR and am still working out my recovery procedures. I'm flying a Telemega gps/telemetry board using a 3 element yagi connected to tablet for the APRS feed. What I'd like to do is get a hand held transmitter to RDF as a backup for the telemetry link. Not sure what the best options are. I'm aware that RDF range is pretty limited with the Telemega once it's landed, but I figure that something is better than nothing and an appropriate ht would allow me to add an independent RDF tracker to my rocket in the future.

    Any thoughts on ht's to consider and which features I need to look for would be much appreciated.

    PT3


  2. #2
    Join Date
    26th November 2009
    Posts
    4,721
    The best unit is an H/T with a true signal strength meter and an attenuator of some kind. Arrow sells them and I think Adept has an attenuator.

    If you want to go on the cheap this one you can cobble together, mount it in a box and secure it to the Yagi with Velcro: http://www.west.net/~marvin/k0ov.htm

    I've used the K0OV attenuator with success and a TH-F6A.

    The cheap Chinese H/T's "DO NOT" have a true signal strength meter! It's either all on with a signal
    or all off.

    Now some say if you use earbuds with a mono convertor plug you can detect the signal strength with your ears but if your hearing is a bit off
    a true signal strength meter is more helpful.

    If you plan on doing APRS tracking later on get a D72A and you can do anything with it on the Ham bands. A TH-F6a might have the potential to do the "animal trackers" on the B band with an appropriate 200Mhz Yagi. I did some preliminary tests but not a rigorous field test. The F6A of course could RDF track the TeleMega.

    If you're going to do RDF and APRS on the Ham bands only and not mess with animal or Com-Spec trackers, a D72A or a Yaesu used
    VX-8GR (only if you can get it at a good price used) would be likely candidates. If the $$$$$$ are unlimited a D74A could do just about
    anything for you and then some but costs are extremely high for that one.

    The thing with the GR and 72 is the ability to put the rocket waypoints on a map live and compute a live navigation solution on the fly.
    If you want the potential to do that, that's the rigs you want to consider. Kurt KC9LDH


  3. #3
    Join Date
    4th December 2017
    Posts
    3
    Thanks Kurt, much appreciated.

  4. #4
    Join Date
    20th August 2010
    Location
    Portland, Oregon
    Posts
    382
    Quote Originally Posted by PT3 View Post
    Any thoughts on ht's to consider and which features I need to look for would be much appreciated.
    One big benefit of learning RDF is just how much range this offers on the ground over APRS or digital telemetry. I've been able to hear RDF tones from miles away, while APRS or digital telemetry is limited to half a mile or less. Find a bit of elevation and the range is even greater; even a few dozen feet can be dramatic. That's why all of our devices offer RDF as a backup to GPS telemetry; it can provide that critical piece of information to get you close enough to receive the GPS data and locate the rocket, in cases where you failed to get GPS data as the rocket approached the ground.

    As far as HT features go, there are two potentially useful ones. The first is a "real" signal strength meter. As others have noted, some less expensive receivers have a meter which is essentially all-or-nothing. That makes it harder to find the peak signal strength when sweeping across the signal. The second is an RF attenuator. That allows you to reduce the sensitivity of the receiver so that when you are close to the transmitter, you can still get some variation in signal strength instead of having the signal 'really loud' in all directions. If you can receive GPS data, that's less important, but if you're just RDF'ing, it's pretty necessary in any kind of brush.

    I've done RDF with inexpensive radios, with those, you have to learn how to distinguish signal strength by the relative amount of tone to noise you're hearing. It's not as accurate as a signal meter, but it is generally sufficient to get an idea of which direction to head; certainly within 45 degrees or so. That has generally been in combination with a TeleBT and android device to get GPS data once we got close enough. I think it'd be hard to get all the way to the transmitter without a reasonable meter.

    What I really like to carry is a radio with good RDF performance and APRS reception. This provides a backup to my TeleBT/Android setup for GPS data (in case the phone dies) while also offering the ability to do RDF in case I need to. That's not a cheap option though; the FT1D that I bought to do APRS development is the most expensive radio I've ever owned.

    Finally, RDF requires a directional antenna. I use a 3 element Arrow yagi, but you can easily build one too. It's nice to have at least a little gain to improve the radio system range. I swap the antenna between the TeleBT and the FT1D if I want to RDF as I want to capture as much telemetry data as I can during flight, and if I can get GPS data from the rocket to my android device, having live maps with that presented makes it possible to navigate for recovery.
    Keith Packard
    TRA 12200/NAR 88757 Level 3
    Altus Metrum Rocketry Electronics

  5. #5
    Join Date
    4th December 2017
    Posts
    3
    Thanks Keith, I appreciate the help. The FT-2DR is a bit out of my price range at the moment, would the F-70DR be worth the trouble?

    On my first flight with the Telemega my tablet froze right after lift off. By the time I got my computer back online, I had lost sight of my rocket and thought I was sol. I ended up following other recovery teams and was able to get close enough to my rocket to hear the Telemega beeping and recovered it successfully. It appears that the Telemega worked flawlessly, the operator not so much. Should it happen again, I'd love to be able to plug the yagi into an ht so that I can pick up the RDF tone and at least get a fix on the general direction. I've used my club's walston tracker and had good luck. If I can expand my setup to get the same functionality from my Telemega, I'd be thrilled.

  6. #6
    Join Date
    20th August 2010
    Location
    Portland, Oregon
    Posts
    382
    Quote Originally Posted by PT3 View Post
    The FT-2DR is a bit out of my price range at the moment, would the F-70DR be worth the trouble?
    Just checking eBay, an FT1DR is about the same price as a F-70DR (around $200). I'm not sure I'd be willing to spend that much for a radio that didn't support APRS; you could play with a baofeng for $25 and see what you thought?

    Keith Packard
    TRA 12200/NAR 88757 Level 3
    Altus Metrum Rocketry Electronics

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