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Old August 31st 03, 02:54 AM
Doug McLaren
 
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Default Question about attenuators ...

I'm pretty new to all of this, so if this is a silly question forgive
me

I thought I'd go ahead and make a 2m Yagi antenna so I can give
transmitter hunting a try. (The plan I'm using is made of PVC pipe
and 1" tape measures ... simple enough.)

The other thing you usually need is an attenuator, to cut the power
when you get close. I've got instructions for one of these too, and
it's pretty simple -- just a bunch of resistors and double throw
switches to activate each stage, all in a shielded box.

The values of the resistor are given without explanation, but I'm sure
those values were chosen to 1) attenuate the signal by the desired
amount and 2) maintain the antenna/radio's impedance of 50 ohms. (I
guess I could verify this with a little math if I felt the need ...)
With about 5 stages, the total cost of parts is $30-$40 at Radio
Shack.

Since you're not going to be transmitting through this attenuator
anyways (it would just release the magic smoke) then why is it
important to keep a good SWR ratio? Couldn't you just take a variable
resistor (with an audio/logarithmic taper) and put it in series or
parallel (which exactly depends on it's value) with the antenna input,
and then adjust as needed until you have a sutiable signal level
again?

Or would the messed up SWR ratios actually make the antenna not
directional anymore or something like that?

(http://members.aol.com/homingin/index.html has lots of stuff on
transmitter hunting, but no answers to this question. The offset
attenuator seems good, however

--
Doug McLaren,
"Can of worms? The can is open... the worms are everywhere."

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Old August 31st 03, 03:49 AM
Roy Lewallen
 
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Default

If you do the attenuation in a single line to your receiver, rather than
in feedlines for individual elements of an array, it won't have any
effect on the antenna's pattern or other properties.

There are quite a few good reasons for having an attenuator maintain a
match -- it makes it possible to have a predictable and repeatable
amount of attenuation, it won't disturb other cascaded objects like
filters or amplifiers, etc. But for your purpose, a variable resistor
should be fine. The only problem you might have is that the amount of
attenuation you can get will be limited by the capacitive coupling
across the resistor. You could use two pots, or a ganged pot (with one
section connected so its resistance increases when the other decreases),
with one in series and the other across the line on the receiver side,
to increase the attenuation range.

Roy Lewallen, W7EL

Doug McLaren wrote:
I'm pretty new to all of this, so if this is a silly question forgive
me

I thought I'd go ahead and make a 2m Yagi antenna so I can give
transmitter hunting a try. (The plan I'm using is made of PVC pipe
and 1" tape measures ... simple enough.)

The other thing you usually need is an attenuator, to cut the power
when you get close. I've got instructions for one of these too, and
it's pretty simple -- just a bunch of resistors and double throw
switches to activate each stage, all in a shielded box.

The values of the resistor are given without explanation, but I'm sure
those values were chosen to 1) attenuate the signal by the desired
amount and 2) maintain the antenna/radio's impedance of 50 ohms. (I
guess I could verify this with a little math if I felt the need ...)
With about 5 stages, the total cost of parts is $30-$40 at Radio
Shack.

Since you're not going to be transmitting through this attenuator
anyways (it would just release the magic smoke) then why is it
important to keep a good SWR ratio? Couldn't you just take a variable
resistor (with an audio/logarithmic taper) and put it in series or
parallel (which exactly depends on it's value) with the antenna input,
and then adjust as needed until you have a sutiable signal level
again?

Or would the messed up SWR ratios actually make the antenna not
directional anymore or something like that?

(http://members.aol.com/homingin/index.html has lots of stuff on
transmitter hunting, but no answers to this question. The offset
attenuator seems good, however


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Old August 31st 03, 04:20 AM
Crazy George
 
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Default

Doug:

To add to what Roy says, I have been able to only get 20 to 25 dB total
attenuation at VHF with a single pot. And, the effect he overlooked is that
unless you use the exact same electrical length cables, the attenuation will
be different for every other configuration. So, if you develop a feel for
knob position vs distance while hunting, it will change if you don't use
exactly the same cables each time.

Haunt the swapfests and evilbay for attenuators. I have seen HP units go
pretty cheap. Be careful of Kay brand, as they are known to have bad
switches when they get old. If you want to learn the innards, that is a
good way, rebuilding a Kay.

I think there was an old HP spectrum analyzer main frame with a model number
like 853 which you can't give away these days. But, it has a nice rotary
attenuator in it, if you find one. Then you just have to dispose of the
rest of the box.

--
Crazy George
Remove NO and SPAM from return address




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