Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: When closing in??  (Read 90160 times)
KD2E
Member

Posts: 233




Ignore
« on: November 02, 2013, 08:30:10 AM »

Have not been T hunting yet, but planning a fox hunt with Cub Scouts.
I had a question.
From what I read, when closing in on a hidden transmitter...very close. It seems you REALLY have to have an attenuator to knock the signal down so the receiver can still find a peak.
When close in, is it possible to use an old analog field strength meter connected to the yagi?
It would seem the low sensitivity they have would be ideal for a strong RF field directional measurement.
...Dave
Logged
AA4PB
Member

Posts: 12995




Ignore
« Reply #1 on: November 02, 2013, 09:54:08 AM »

The problem with a field strength meter is that it is broad-band so you could be receiving a signal other than the fox.
Logged
WB6BYU
Member

Posts: 13489




Ignore
« Reply #2 on: November 02, 2013, 03:22:07 PM »

It depends on how strong the transmitter is.  You may find that your FSM doesn't give
much indication until you are only a couple feet away from a 1W transmitter.  You'll
have to try it with your specific FSM, transmitter and antenna and see for yourself.

I like to use transmitters in the 10mW to 50mW range for hunts in the park - that may
not be enough voltage picked up to overcome the knee voltage of your detector diode.
At half a watt you may see an indication a couple feet away, depending on the meter.

And, yes, you could have problems if there is another strong transmitter nearby.  But
if you are hiding for the Cub Scouts, you can probably choose a spot that isn't beside an
AM BC station or a repeater site.  Because the FSMs tend not to be very sensitive, you
wouldn't need to be a long ways away to avoid problems.

Adding a single stage of amplification, either at DC or RF, will increase the distance over
which you can detect the transmitter.  A single transistor biased just slightly above cutoff,
arranged to read collector (or emitter) current as the base is driven slightly more into
conduction on the positive peaks of the RF, may be enough, otherwise a tuned RF stage
(for selectivity) followed by a detector and op amp can probably provide enough gain to
the point where pick up of other signals becomes a problem.

One thing we did for ELT transmitters (~50mW AM on 121.5 MHz) was to build a crystal set, since
they easily detect the AM, you can simply aim the antenna for loudest signal, and they had
no battery to go bad while waiting to be needed.  With these we could hear the signal about
200 feet away, and at 5 feet or so the signal was painfully loud in the headphones.  Again, a
single stage of amplification could be added for longer range, possibly configured so that the
battery was only connected when the headphones were plugged in.   AM detection works
pretty well for FM signals if you have a narrow enough bandwidth, but you probably won't get
that with a VHF crystal set, so this might not work as well with an HT, but if you can provide
an AM signal on 2m it becomes more practical, and you can adjust the output power for a
reasonable working distance.
Logged
KV4BL
Member

Posts: 78




Ignore
« Reply #3 on: November 25, 2013, 09:33:59 AM »

Dave, please forgive me if I am pointing out the obvious, but one neat strategy when foxhunting is to put your receiver in VFO mode and tune off 5, 10, or more KHz from the frequency the transmitter is operating on.  This acts as a field-expedient attenuator and will allow for finding peaks as you get closer.  Hope this helps.

73,

Ray   KV4BL
Logged
AA4PB
Member

Posts: 12995




Ignore
« Reply #4 on: November 25, 2013, 10:24:32 AM »

You don't to be next to a repeater or broadcast station to get a signal on a FSM, especially if you add a preamp to it. I expect that other people looking for the fox are likely to have HTs, cell phones, etc. with them.
Logged
W4CNG
Member

Posts: 179




Ignore
« Reply #5 on: December 26, 2013, 05:10:56 PM »

I used a cross-band HT when setting up foxes years ago.  I would be within 1/2 mile of the fox in a box and exernal antenna.  The DF'ers would find me and claim found fox.  Problem was I was not on the Fox frequency.  They then had to go and do some more fox hunting.  With this mode I could key it and talk to them for as long as I needed to.  Before the crossband HT, I used a 1 Watt HT keyed by a Repeater ID unit set to TX a long call every 6-8 minutes.  All antennas were on the ground and near or on metal fences.  This makes for multiple signal tracks to see.
Logged
VU2NAN
Member

Posts: 253




Ignore
« Reply #6 on: January 08, 2014, 01:25:59 AM »

Have not been T hunting yet, but planning a fox hunt with Cub Scouts.....From what I read, when closing in on a hidden transmitter...very close. It seems you REALLY have to have an attenuator...........................
Dave

Hi OM Dave,

Use an offset attenuator!

http://nandustips.blogspot.in/2011/02/fox-hunt-attenuator.html

73

Nandu.
Logged
N4NYY
Member

Posts: 4818




Ignore
« Reply #7 on: January 12, 2014, 06:35:48 AM »

Have not been T hunting yet, but planning a fox hunt with Cub Scouts.
I had a question.
From what I read, when closing in on a hidden transmitter...very close. It seems you REALLY have to have an attenuator to knock the signal down so the receiver can still find a peak.
When close in, is it possible to use an old analog field strength meter connected to the yagi?
It would seem the low sensitivity they have would be ideal for a strong RF field directional measurement.
...Dave

I have done one. From what I understand, when you are close (if using an HT), remove the antenna and use is a small paper clip or no antenna. The overload will point that you are way close.

Keep in mind that I only did one, and we finished dead last. Ironically, we ended up being so damn close in our first attemtpt before he stopped transmitting, and then in an effort to find him, we drove way too far out! LOL. But that is what the experienced fox hunters told us.
Logged
N6JSX
Member

Posts: 217




Ignore
« Reply #8 on: January 25, 2014, 10:51:19 AM »

There are many techniques to use to accomplish your needs. However, the common factor in success is YOU need to practice and learn/understand the limitations of YOUR HT/radio.

The cheapest to learn is the body-shielding sniffer method where you use your body to create the attenuation of the TX signal to the HT. Providing you are not in a severe multi-path location you put the HT against your belly (fatter the better  Tongue) minus the HT duckie and slowly turn 360 degrees. If you hear a null/reduction in signal/audio the TX is behind you - 180degrees from where your facing. Or dead-reckon the edges of degrees in your rotation (where you can hear/see the signal) estimate the degrees divide the null in half and proceed 180 for a relative bearing to the T.

You have turned a full 360 and the signal is the same with no null - then change the HT frequency by going off the prime by steps of 5/10/12.5 kHz. 15kHz is typically to far outside the HT bandpass.

Another attenuation technique (if you have a dual band HT) is to program in the 3rd
harmonic 146.565 = 439.695 MHz, this will offer about 30-40db attenuation.

When I'm mobile hunting (to insure I do not spoof myself  Roll Eyes) I place a HT (minus the rubber-duckie) set at full volume and just squelched quite on my vehicle dash. When this HT starts sounding off I know I'm close, within ~1/4mi/ in the beam path ~1/2mi. Grin  Check out my YGroup RDF-USA for more info.  Shocked
 
Logged

Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!