Searching with a Yagi

[JordanT]

Just got my BRB beacon running and was trying it out in the house. The signal was strong enough that even without an antenna, the Baofeng was picking up loud and clear. So I hooked up the offset attenuator, tuned +4MHz, and got some rejection. It seems weird, though, that when pointed directly at the beacon from ~20-30' away, I would get nothing, but when I sweep the yagi axis away by +/-20 degress or so (e plane?...horizontally) I get a good strong signal. The yagi was built from an online tutorial, subbing wood for foam core (since I can shape wood).

I'm sure this is pretty simple, but I can't quite figure out why I would have a dead spot in what should be the "best" direction. (I'm hoping that strapping the attenuator on the beam isn't causing issues...though I suppose now I should take it off just to be sure!)

 

[WillMarchant]

Is that a 9 Volt battery in the middle? It's a powered attenuator? Yeah, I'd get all that extra harness out of the way and see what happens then...

 

[JAL3]

I just took down a UHF TV antenna a few days ago and was going to throw it out. I'm wondering, though, if it would be of any use in locating rockets or have any rocketry use at all for that matter.

Be advised that to me, 80% of the first post was gobbeldygook. Keep it simple, please.

 

[JordanT]

Well, as convenient as the physical location of the battery was, it was clearly a bonehead RF move. Of course, the only partially fixed the problem. Now I get strong signal pointed directly at the beacon. But from ~30', I get signal from every direction. I'm going to have to play some more with this, because I was getting good signal in most directions, even when I was 60 MHz off the beacon frequency.

(an exercise for tomorrow, since I just pulled the + lead from the 9V off the board - oops)

edit - I can't imagine a TV antenna providing much in the way of use; the TV bands are exclude the HAM bands popular with beacons, and don't go as high as the 900MHz license free band some transmitters use. Then again, at least 60% of what I said above I have only a tenuous grasp on. Had you told me 2 weeks ago that the antenna I was going to build was going to have none of the elements actually connected by wires, I'd have though you insane (or an RF engineer, but that's somewhat redundant).

 

[troj]

When you says you're "60MHz off the beacon frequency," just to make sure we're all on the same page....

You've set your attenuator to 60Mhz. Did you then also change your radio's frequency to 60MHz off in that direction?

-Kevin

 

[troj]

I just took down a UHF TV antenna a few days ago and was going to throw it out. I'm wondering, though, if it would be of any use in locating rockets or have any rocketry use at all for that matter.

Be advised that to me, 80% of the first post was gobbeldygook. Keep it simple, please.

Nope, won't work -- the UHF antenna is setup for a different wavelength. It's also designed to be more unidirectional.

-Kevin

 

[JAL3]

It guess it will remain on the rubbish pile waiting for pickup.

Would an FM be of any more use? I take one of those down next.

 

[troj]

It guess it will remain on the rubbish pile waiting for pickup.

Yeah, when I insulated our upper attic, and discovered a TV antenna, it came out in pieces and got tossed in the recyle bin.

Would an FM be of any more use? I take one of those down next.

Nope. Wrong wavelength, again.

Antennas are very specific to the frequencies they're designed for.

-Kevin

 

[JAL3]

Thanks. It was worth a shot.

 

[JordanT]

When you says you're "60MHz off the beacon frequency," just to make sure we're all on the same page....

You've set your attenuator to 60Mhz. Did you then also change your radio's frequency to 60MHz off in that direction?

-Kevin

It's a 4MHz offset, so i just worked up the spectrum from 433.925 (beacon frq) 4 MHz at a time, trying to get a lousy/directional signal. I gave up at 493.925 MHz (+60MHz), which should have been five billon dB down (or is it not like a resonance with higher freqs causing higher attenuation?). I was running out of room in th band of my receiver. . I'm going to change the beacon freq off 0.2 Hz tomorrow - maybe 1 hz - and turn down the power so I can characterize it better. I may also be more careful with (attentive to) the polarization of the beacon.

 

[JimZNJ]

If you are truly testing indoors, you are subject to all sorts of reflections of the signal. I don't know the layout of your house, but if you didn't have a line of sight path to the beacon then you might have found a stronger signal on a reflection. I know when I first received by BRB beacon I tried to test it indoors and it was impossible to get a proper directional fix on it. Once I took it outdoors it got so much easier.

I also use an offset attenuator. In my case it is a commercially made one from Arrow Antenna. I tune my receiver 4MHz higher than the beacon frequency. The attenuator I have is adjustable. Besides attenuating the signal to more discernable levels, it also ensures the signal you are receiving is coming through the antenna and attenuator and not just leaking through the case of your receiver.

If you still have problems after trying it outdoors, I'd look into moving that 9V battery elsewhere. It could be adversely impacting the pattern of your antenna. Programming the BRB beacon down to a lower output level during your shorter range testing is a good idea. You'll want it cranked back up for flight since once landed the ground will absorb a lot of the signal.

The BRB beacon is very useful and I've had great success using it with an offset attenutator and a yagi to find rockets solely by ear (no S meter).

Good luck!

Jim Z

 

[markjos]

I just want to echo Jim's comments above:

You've got a bunch of variables in this test - a homebrew antenna (untested?), and maybe a homebrew offset attenuator (also untested?). It's tough to test one without knowing that the other is okay.

70cm transmissions are very prone to reflection, and indoors could be a nightmare. When our local group of hams puts on a foxhunt, and someone suggests a 440MHz fox transmitter, everyone just groans. Even outdoors, an area with buildings can produce reflections that are just maddening. Fortunately we fly rockets in pretty wide open spaces, and while we'll get the occasional reflection off of a barn or silo, it's not so bad. Go test this outside, away from structures, if possible.

If this is a common offset attenuator circuit (like Joe Moell - K0OV's version https://www.homingin.com/joek0ov/offatten.html), it should allow you to adjust how much of the 4MHz signal is being mixed with the beacon signal off the antenna. Keep the radio tuned to exactly 4MHz above or below the beacon frequency, and adjust the attenuator level.

You can test the attenuator with any appropriately tuned antenna first - even the rubber duck from your radio - and not worry about the yagi. Similarly, you should test the antenna without the attenuator. That yagi design *will* have some narrow lobes of gain off to the sides - not as strong as out front, but they're still there. If there are any problems with the antenna construction - element spacing, metal batteries in the middle, etc., then the pattern is not going to be what's expected.

Keep trying, and keep asking questions. You'll get this working, and you'll really be satisfied when it helps you walk right to your rocket.

Mark
NAR/TRA L2
Central Illinois Aerospace

 

[JordanT]

Thanks for all the suggestions. Once I get my "home" work done to day, I'll take the beacon out to the local golf course. It's 17F out there right now, I doubt there will be golfers!

It is a K0OV offset attenuator, but I haven't put it in a box yet. Right now I'm adapter poor, so putting the rubber duck on the attenuator isn't an option (SMA on the HT and rubber duck, BNC on everything else, one home-made SMA-BNC adapter). Actually, the beacon is the only known signal as the closest repeater I know of is probably 30+ miles, and I don't know any other hams in the area. I suppose I could try to pick up a satellite, but since I've never done that, it seems like another dead end.

Taking the battery out from in front of the collector makes a noticable difference in the received strength. And I suspect I mis-read the OA information - I thought you would get and additional 30-60dB down for *each* 4MHz you went off band, but that sounds like it isn't the case. I'll go back to +4MHz and re-start from there

 

[JordanT]

Well, I didn't get out today, but I did find time this evening to put the OA into a box and wire it up with a switch. Going back to living room tracking and using the proper (4MHz) offset, I had good luck. Definitely something I'm going to need to practice with, but everything seems to work like a charm.

Next up...working with the GPS version.

 

[troj]

Well, I didn't get out today, but I did find time this evening to put the OA into a box and wire it up with a switch. Going back to living room tracking and using the proper (4MHz) offset, I had good luck. Definitely something I'm going to need to practice with, but everything seems to work like a charm.

Next up...working with the GPS version.

FWIW, when you use the GPS, you don't need the attenuator, or the Yagi. The rubber duck will work just fine.

-Kevin

 

[sti_ffy]

FWIW, when you use the GPS, you don't need the attenuator, or the Yagi. The rubber duck will work just fine.

-Kevin

When using the rubber duck, do you actually end up using the last position that was transmitted before the rocket reaches the ground? I ask because when I used the BRB GPS beacon in my nosecone at RG13, the slight rise in terrain between me and the rocket was all that was required to completely attenuate the signal. I could get nothing at all on my TH-F6A even with a 7-element yagi and the squelch wide open. I had to get within ~100 yards of the beacon before I was sufficiently clear of the terrain that the beacon's signal began to quiet the receiver (never mind strong enough to use the offset attenuator).

 

[bigredbee]

That's what I do. I never use a yagi when flying a GPS

 

[troj]

When using the rubber duck, do you actually end up using the last position that was transmitted before the rocket reaches the ground? I ask because when I used the BRB GPS beacon in my nosecone at RG13, the slight rise in terrain between me and the rocket was all that was required to completely attenuate the signal. I could get nothing at all on my TH-F6A even with a 7-element yagi and the squelch wide open. I had to get within ~100 yards of the beacon before I was sufficiently clear of the terrain that the beacon's signal began to quiet the receiver (never mind strong enough to use the offset attenuator).

Yep. My radio holds the last position it received, until it gets another packet it can decode.

That will get you close, and as you get closer, you'll eventually begin receiving updated packets.

-Kevin

 

[Dwatkins]

That's what I do. I never use a yagi when flying a GPS

How high have you had success with this? I have been to 20k this way but have wondered were the rubber ducky would fall out.

Dennis

 

[CarVac]

If you have dual deploy, it'll come back within range before it hits the ground unless something goes badly wrong or you're doing an extremely high flight.

 

[bigredbee]

I've seen it used on flights to > 50K' AGL with just the rubber duck. I'd say you're good to about 80K before you'd need a gain antenna on the receive side.

Greg

 

[JordanT]

Just an update: got a winner tonight on the GPS to acquire and determine position. I either have the cheapest or most expensive hand held tracking unit available:

BeeLineGPS

Baofeng UV-5R ($40 shipped from Amazon)
Packet Pad for iOS ($2.99)
Google Earth for iOS (free)
iPhone5 (Free or $699 or $1350, depending on how you count it)

I'll admit I spent another $20-30 on various and sundry connectors and programming cables for the UV-5R, but I really didn't need it for the tracking. I just entered the frequency from the keypad and I was off to the races.

Packet Pad listens through the regular microphone port and decodes aprs on the fly, printing the data transmitted by the BeeLineGPS on the phone screen. From there, you can copy the coordinates, paste them into google earth (performing a bit of editing to meet the google earth format), and *bazinga* you fly right to the spot where your transmitter is.

It goes something like this:

[video=youtube_share;zJMWm8M59R4]https://youtu.be/zJMWm8M59R4[/video]

 

[Buckeye]

Very cool.

 

[degreaser]

You've built a cheap yagi. It's an excellent antenna. I've built several. I've used a dual band 2m/440MHz version to have QSO's with Oscar satellites with a handitalkie. One thing I do on mine is add a bit of extra length at the reflector end of the antenna. This gives you something to hold onto without affecting the antenna characteristics too much.

An attenuator isn't absolutely necessary. Use the yagi when the Fox (rocket) is a fair distance away. Then switch over to the rubber duck antenna, using your body to shield the antenna. When you get closer, take the rubber duck off and use the radio without an antenna again using your body as a shield.

 

[JNUK]

Some time ago I build a similar antenna.

The black box on the photo is a simple receiver.

Here are some details of the design: https://www.rocketryforum.com/showthread.php?22221-Simple-RF-tracker
The tracker saved my model on at least 5 occasions. Would be a total loss without it.


20...30' away may not be a sufficient distance to asses directivity of an antenna in 400 Mhz band. Although it is not the near field, but not the true far field as well. Something in between. Generally it is OK but all sorts of strange things can happen. Especially if antenna is surrounded by various not radio transparent or conductive objects. The antenna diagram can be distorted quite significantly.

 

[JordanT]

Yup - that's just like the yagi I built. I "designed" the handle on mine for balance, not RF (again, oops - I may re-engineer that). Oddly enough, the rubber duck that came with the Baofeng is actually very good on 70cm. I also picked up a different app called PocketPacket. The reviews are not great, and it doesn't appear to be updated recently, but it coupled acoustically very, very well with the HT and I monitored my wife as she walked the dog around the neighborhood tonight. The app allows you to filter signals by call sign so that only the APRS packets from my beacon show up. I lost her behind a few of the hills, but that shouldn't be a big problem for getting coordinates while under chute.