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another radio question

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cynic

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I'm thinking of building a simple radio beacon and direction finder. This would be a backup locater for my high altitude balloon hobby :)

From my quick reading on wikipedia it seems like a loop antenna might work:

Assuming I used a FRS or GRMS radio I would need to hack the antenna on one of the radios to a 2.5 inch loop - Frequency Wavelength Calculator

From the radio post below I was thinking:

Given the 25 inch wavelength --

Take a PVC pipe with at least a 25 inch diameter
Put a cap on one end and some RF shielding on the outside
Place my receiving radio inside the tube and in theory I would get reception only when pointed at the beacon?

Thoughts?
 
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I have years of experience in tracking Aircraft Emergency Locator Beacons (121.5Mhz) and Fox-Hunting (Bunny-hunting) on the 2m Ham Band (146MHz). Best frequency to use for RDF is ~100 to 200Mhz. Below that the DF antenna is too big. Above that, the radio signals bounce around too much, so it is hard to separate the direct signal from a bounced signal. Don't screw with a loop; it has nulls in two directions. Use a 2 or 3 element Yagi. Best receiver for DFing is an AM receiver.
 
For direction finding at 460Mhz (ish), a loop antenna sounds a poor idea, a loop antenna inside a "bucket" sounds even worse.
I would be inclined to use a yagi antenna, which could be quite compact but have useful gain and directivity.
However, an FM receiver is a really bad idea. As Mike suggested, an AM receiver with a meter on the AGC would be ideal, but where you would get one for that frequency, other than make it yourself, I dont know.

Consider the possibilities of using a low frequency, 1Mhz (ish).
AM receivers are plentifull, the internal ferrite rod (loopstick in the USA?) antenna has very good directional properties and a signal strength meter is easy to add to the AGC line.
A simple transmitter is very easy to build.
The problems:
The ferrite rod antenna has two nulls* which is ambiguous for direction finding, but adding a short rod antenna will make the response into a cardiod with one null* for unambiguous direction finding.
The antenna on the transmitter will need to be quite long in order to radiate a strong enough signal. (How long is "quite long" I would not like to postulate without doing a test.

* It is usual to take DF bearings using the null in the antenna response, rather than the peak. The reason being that the null is much sharper and gives a more precise reading.

JimB
 
I have years of experience in tracking Aircraft Emergency Locator Beacons (121.5Mhz) and Fox-Hunting (Bunny-hunting) on the 2m Ham Band (146MHz). Best frequency to use for RDF is ~100 to 200Mhz. Below that the DF antenna is too big. Above that, the radio signals bounce around too much, so it is hard to separate the direct signal from a bounced signal. Don't screw with a loop; it has nulls in two directions. Use a 2 or 3 element Yagi. Best receiver for DFing is an AM receiver.

I built a collapsible 3 element yagi for DF hunting, using wooden dowel, rubber bands, and 300 ohm blanced feeder.

But I used to DF hunt on a motorbike :D
 
Hmmm.... I'd like to build something simple and effective. One of my concerns is that locating the beacon using something like a yagi antenna may take some skill and I can see that using a beacon with a short wavelength could be problematic as the signal will bounce around.

Any suggestions for a different solution? I thought about a small infrared beacon but I'm not sure how you could come up with a cheap infrared monocular. I'm trying to keep the entire cost to maybe $30.00

I can get off the shelf radios of about $20.00

An audible beacon (as in just a loud noise) seems like it would not be effective beyond a few feet and would be horribly annoying :)

Thoughts?
 
Hmmm.... I'd like to build something simple and effective. One of my concerns is that locating the beacon using something like a yagi antenna may take some skill and I can see that using a beacon with a short wavelength could be problematic as the signal will bounce around.
...


If you think that DF with a Yagi takes skill; you are right. But it is a lot easier than trying to use a loop.

Look at the folks that put radio collars on wildlife. They all use Yagis, even when DFing from an aircraft, where they mount one on each wing-strut and just use a coax switch to compare right side to left side. The radio collars use a simple one-transistor VHF crystal-controlled oscillator which transmits in bursts, for battery saving, and to help identify it audibly in the receiver audio.
 
bluetooth

Hmmm... **broken link removed**

"An Inexpensive Bluetooth-Based Indoor Positioning Hack"

This article essentially proposes the 'receiver in a tube' hack I thought of trying with a GMRS radio.

It would be easy to test. The wavelength on a bluetooth device is about 5 inches as opposed to the 25 inches you would get with a GMRS radio.

I may tinker with this hack a bit and see if it does indeed work.
 
If your balloon never strays further than about 10 meters from you, bluetooth would be perfect. ;)
 
ummm.... yeah, that might be a problem. I still need a cheap way to radio locate the balloon as a backup and all signs point to that not being possible. Drat.... You would think somone would make a cheap automatic direction finder but it appears if I want one I'll have to make it from scratch.
 
A scanner with an AM receive mode, a hand-held Yagi, and a one transistor transmitter, which is what I told you about 17 posts ago...
 
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A scanner with an AM receive mode, a hand-held Yagi, and a one transistor transmitter, which is what I told you about 17 posts ago...

Hmmmm.... Is there a way to build a display with LEDs or something similar for a yagi antenna?
 
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Just use your ears. If you tune the receiver slightly off frequency, you can use the loudness of the receive audio to judge when the Yagi is pointed at the transmitter. The front to back ratio of a three element Yagi is about 20db, which is good enough to localize the signal quickly. The 6db forward gain of the Yagi helps in initially aquiring a weak signal. This is much faster than staring at a meter. Headphones to block out external noise help.
 
yagi

Perhaps something like this? - Kent Electronics, WA5VJB

GMRS radio is in the 462Mhz range. He makes a log peridoc antenna in that range but not a yagi. Is a yagi significantly better for direction finding?

Thanks
 
A Yagi is easier to build than a Log Periodic. A Log Periodic has just one virtue, wide bandwidth, or to put it another way, it works equally poorly at a lot of frequencies ;)
 
That was essentially my wikipedia interpretation of it as well. What about the concept of the Yagi on a PCB?

I'm guessing he will build one for the 460mhz range if I just ask.
It seems like I could probably cobble up the parts on my just as well though.
 
Get hold of an ARRL Radio Amateur's Handbook or the ARRL Antenna Manual (any year). Your library may have one or the other. There are designs for 146MHz and some of the higher Ham bands. The beauty of antennas is that their dimensions can be scaled by the factor [design frequency]/[new frequency].. For example, a Yagi designed for 146MHz can be scaled to 180MHz just by multiplying all the dimensions by 146/180. Works like a charm. If making a large jump, it is necessary to scale the diameter of the elements.
 
That's you? Funny, I had been through those pages. The link I posted just had an easier for me to read circuit diagram :)

The cool thing is, I already have the GMRS radio on my balloon transmitting coordinates every few minutes; as a backup, in case the primary GPS goes out, the secondary GPS fails I'd be able to radio locate my equipment and that is my interest. It's a backup that doesn't require any additional payload weight.

I'm not sure if I'll just get a yagi antenna which is pretty easy to build or go nuts and build the tdoa circuit.

In any case, thanks for the pointers. I'm surprised there is not an off the shelf TDOA circuit you can just plug in as it seems the length of the antenna would not be specific to the frequency band in use.

I just noticed you fly a cessna. I'm a CFII / MEI / Former corporate pilot.
 
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...I'm surprised there is not an off the shelf TDOA circuit you can just plug in as it seems the length of the antenna would not be specific to the frequency band in use.

I just noticed you fly a cessna. I'm a CFII / MEI / Former corporate pilot.

3300 hrs in C182s and taildraggers. I used to be a Search Mission Coordinator and SAR Pilot for CAP. Have lots of hours tracking ELTs from aircraft. The aircraft Homing DFs mostly use an electronically switched directional antenna array which creates an AM modulation on the received signal, so it uses AM receivers.

The TDOA system creates a phase modulation on the received signal, so it works best with FM receivers. There are both commercial AM and FM DFs available.
 
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