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Author Topic: Offset Attenuator Not Attenuating  (Read 7179 times)

K6OI

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Offset Attenuator Not Attenuating
« on: June 05, 2008, 08:51:37 AM »

My offset attenuator built from the Homing In website is no longer functioning correctly.  I used to get enough attenuation to completely null out the signal within 5 feet of the fox but now the signal pegs even @ 20 feet away.  The pot adjustment seems to have very little affect on the signal strength.  I replaced the diode with no change.  It's not a very complicated circuit so I'm not sure what can go wrong.
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K0OV

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Offset Attenuator Not Attenuating
« Reply #1 on: June 05, 2008, 09:21:54 AM »

I have to ask the obvious question:  You have your receiver tuned to the offset frequency, not the transmitter frequency, right?

If it's tuned to the offset frequency, then the only thing that I can think of right now that might cause this would be a bad connection on the ground side of the potentiometer.  That would put nearly full oscillator signal into the mixer no matter what the pot setting.

Are you using the Marvin Johnston board or did you build it from scratch?

If you can't fix it, bring it to our next ARDF session and I'll take a look at it.  For anyone interested, our next session will be June 14 in north Fullerton.  More information at www.homingin.com
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WB6BYU

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Offset Attenuator Not Attenuating
« Reply #2 on: June 05, 2008, 01:26:19 PM »

One problem I've found is that the oscillator will stop
working when the batter drops to about 7.2V or so (depending
on the voltage regulator.)  My box has LEDs to indicate
the different modes and they still light at this voltage.

But there are a lot of variables - are you using it with
the same radio that you did before?  Do you offset the
frequency in the same direction?  By the same amount?
Using a 4 MHz oscillator, many radios have different
sensitivity at 142 MHz or 150 MHz, and tuning off by a
multiple of the offset (for example 12 MHz) should give
additional attenuation.  My old TR-2600 switches in a
wide band front-end circuit above 150 MHz that makes the
radio more prone to overload from the fundamental signal.

So make sure you have the frequency offset correct and
try a new battery.  If that doesn't work, take Joe up
on his offer to look at it.  (Unless you are in Oregon
or Washington, in which case I'm probably closer.)

K6OI

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Offset Attenuator Not Attenuating
« Reply #3 on: June 05, 2008, 02:07:53 PM »

Hi Joe,

Thanks for the reply and the troubleshooting hints.  I found the problem.  Apparently I inadvertently moved the attenuator too close to the feed point of my homebrew yagi antenna.  I appears that the 4Mhz signal from the oscillator coupled to the antenna and completely swamped the DFing receiver.  I moved it back a few inches from the feed point and all is well again.   Besides the homebrew attenuator and yagi, I am also using a fox built from plans written up in the May 2005 QST.  It uses a Ramsey 433 Mhz data transmitter module.  For the looping fox transmitter signal, instead of building the ISD voice chip programmer called out in the article, I simply used a 30 second voice recording module from Radio Shack.  I am using this setup to demo fox hunting to a group of 100 elementary aged kids at my church tomorrow night.  Thanks for the invite to the next ARDF event.  I went to one a few years back also sponsored by Dave’s group at El Dorado Park in Long Beach and had a blast.

Regards,

Tedd K6OI
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K0OV

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Offset Attenuator Not Attenuating
« Reply #4 on: June 05, 2008, 07:49:14 PM »

Glad you found the problem.  It probably isn't a good idea to put the attenuator right on the driven element anyway.  I worry when people put anything in the space between reflector and driven element, and also when they hold the antenna by the boom between reflector and DE.  I'm sure that this proximity adversely affects the antenna performance, especially on 70 cm.  I encourage people to mount attenuators and receivers behind the reflector and to use some sort of handle so that the holding hand isn't inside the beam.

It's great that you are doing the demo at church and I hope that the kids can try it for themselves.  On Saturday, we'll have a dozen or more transmitters in a park in Long Beach for the Scout-O-Rama.  If it is like years before, there will be so many Scouts that it will keep five hams busy all day giving the kids a chance to hunt two or three of them each.

If any of your kids show an extra amount of interest in foxhunting, tell them about the ARDF events we have here in soCal and encourage them to come out.  That's how we got Monique, the 12-year-old girl who went to the USA ARDF Championships last year.
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