HELP! me figure out RDF on the playa

[Fdog]

The background info: I earned my Amateur Radio (ham) license just so I could do RDF. My equipment is:

• BeeLine 70cm low power tx
• Yaesu FT-60R
• 7-element 70cm Yagi from Arrow

I've played around with this quite a bit at home. I can generate reliable bearings from anywhere in the Reno valley, up to 8 miles away at least. It's easy to find the BeeLine in the classsic Fox-hunt style, even in near-field searching. So with a couple of weeks training, I was excited that I would not lose another rocket in the Black Rock desert. Ha ha...

The First Problem

As soon as the rocket lands, the nosecone ends up on it's side. This places the antenna about an inch from the playa. And as we all know, if you dig down about a foot you'll find water...soooo... the antenna is in the worst possible configuration, parrallel and close to a giant ground plane.

From a mile, even with the high-gain 7 element Yagi pointed directly at the BeeLine, you can't even hear it. At 1/4 mile, you can hear it, and about 1/8 mile it is a high enough signal that there is no discernable null or peak signal. (Not to mention at 1/8 mile you can see it and just walk over and pick it up!)

So, once the rocket has landed on the playa, the RDF is pretty useless if it is more than a half mile away.


The Second Problem

So, we switched gears, with the intent of using the Yagi to find a bearing before touchdown. This way, I reasoned, we could run down that bearing with our RZR's and as we approached the rocket, we would start to pick up it's signal. This would tell us left or right, and we'd find it.


More ha ha.

While airborne, the antenna is at its best, so much so that even the low power BeeLine overpowers the S-meter. Even when the Yagi is aimed 90 degrees off of the known bearing and the rocket is over a mile away, we get no input for the bearing. The S-meter is pegged in every direction. And, as soon as it lands, that's it - nothing. Silence. Darn.


What we do now

In all honesty, we really don't have a huge need for the BeeLine exactly; we watch the rocket through landing, get a visual bearing, and run out on that. Then, when we can hear the BeeLine, we use the Yagi to determine if we need to turn left or right, which is a good exercise, since the visual has been good enough to get us within easy sight of the rocket anyway.

BUT - eventually we'll be flying stuff that we will lose sight of, and it would be nice to have our act together before then, which is why we're practicing now.



My current plan is to use an offset attenuator (in fact, one is on the way from Arrow) to reduce the signal so we can get a decent airborne bearing. Still, I would rather learn how everyone else does it.

Any ideas?


All the best, James

 

[troj]

The playa creates special, unique problems that I've not encountered anywhere else.

I'm replaying information from Ken Biba, as well as I can -- he explained it back in April, so I may be a bit off.

Part of the problem is the nature of the playa itself -- the chemical composition is such that it absorbs radio waves more than typical soil does, rather than reflecting them to any degree.

The second problem is how incredibly flat it is. It has to do with Fresnel Zones, and the fact that a transmitter and a receiver on a flat surface (hello, playa!) will have a dramatically reduced range.

In a nutshell, transmitter range is horrific on the playa. It's the nature of the beast.

-Kevin

 

[UhClem]

A lot of it is Walston specific but there is a lot of good information in a series of 1999 rec.model.rockets posts from Sue McMurray. A copy can be found at the bottom of: https://www.vernk.net/RocketFinding.htm


Even with a directional antenna it can be hard to find the direction of the signal peak. The nulls of the antenna pattern are usually sharper than the peaks so finding the nulls on either side of the peak and then bisecting the angle can help.

 

[Fdog]

Thanks for the references, David. Sue's article is certainly entertaining! Alas, it is all things we do already, I was hoping for a big "Ah HA!" moment.

What I've encountered (on the Black Rock) is that it's either feast or famine - when the rocket is in the air under parachute, the signal is full strength on the S meter no matter which way I point it; as soon as it touches down, it can't even be heard.



All the best, James

 

[tfish]

James..From your screen name and avtar I should have figured you out sooner....did you loose a rocket too? The attenuator will help alot. It helps narrow down the rocket to about an eighth as compared to witout it. Keep dialing it in as it gets closer to the ground..once you loose the signal ..take your foot and draw a line in the dirt in the last direction that you 'think' it was. From there get a reference point way out...say those blond mountains or something. The higher your rocket goes the further out they land..for the most part. Your search area needs to be wider the farther out you go..Next weekend I'll be looking further out for my 'misplaced' rocket. I think it's up in the dunes.

Tony

 

[MCS]

GPS??

 

[Neuport]

I don't have experience on the playa but I do have the BRB 70cm HIGH power, the 7 element yagi from Arrow, and the offset attenuator. I have had 100% success tracking a handful of flights, some quite far out. I think you are working in the right direction. Here are some pointers from my experience.

1. Before launch I set the attenuator so that it is 1 or 2 bars off full strength when aimed directly at the rocket on the pad and then it drops off sharply to the sides.
2. I track the rocket in the air and am able to get a good bearing to the landing location.
3. Typically the signal disappears once the rocket is on the ground.
4. By walking/driving the bearing I eventually get the signal again. The range is highly dependent on terrain and for me has been between 1-3 miles. But, even if the range was significantly shorter I would have picked it up eventually because I started out on a good bearing.
5. If you aren't getting anything try turning the squelch off. At Airfest, I had driven a ways with the antenna out the window stopping occaisionally to get out and sweep with no signal received. I stopped on a rise, got out, and turned off the squelch and picked up a very weak audible signal buried in the static that allowed me to verify my direction and move on with confidence. This signal could not break even the lowest squelch setting nor move the meter at all. On my Yaesu handheld there is a monitor button below the PTT that I can press and hold to monitor without squelch.
6. Be aware of the duty cycle you have the BRB set at. E.g. if it is only beeping every 5 seconds slow your sweeps down to a deliberate 5 seconds on each say 10 degree heading to make sure you don't sweep passed it.
7. If you start out with the attenuator, be sure to disable it when you are searching for a lost signal. If in doubt, remove the attenuator and retune the radio to the actual BRB frequency. Put it back on if the signal is getting swamped at the end.
8. I also have the fox hunting loop which is fun to play with around the house but I haven't needed it in the field yet. I suppose if the rocket went down in corn or high grass it might be useful since I can combine it with the attenuator and, at home at least, pin point the rocket within a foot or two at close range.

 

[Fdog]

Thanks for the tips Tony & Neuport. Looks like the attenuator is the missing link so to speak.

The step-by-step is very handy, Neuport. I already have the beep set for 1 second, which I am sure decreases battery life, but it's worth it I think.

MCS, I could go straight to GPS I suppose. Since RDF is a fallback for a GPS transmitter, I'd like to learn RDF really well first.


Thanks, all!


All the best, James

 

[ChuckH]

GPS??

GPS units for all! Mark is buying! Really Mark, you shouldn't have