[VHFcontesting] Transmit Attenuator

[Rogers, Ron]

John:
I guess I assumed everyone would intuitively know to install the coax
attenuator in the TX path only and not the TX-RX path. But, thanks for
adding that point. But, a number of UHF and Microwave transceivers do come
with separate TX and RX input-output jacks for a transceiver and you add the
external TR relay so it would be up to the user to determine where to
install the TX drive attenuator.
 
But, some prior experience to share with everyone.....in the 35 plus years I
have been building my own contest equipment and repeaters I have learned the
hard way what are reliable designs and which ones have caused me the most
grief when they have failed. Yes, I have built resistor "T" pad TX
attenuators and have had the unpleasant experiences when they fail.....right
in the middle of a contest and they blow up a critical piece of equipment.
The calculated resistor values needed for the "accurate" attenuator usually
are NOT available in standard carbon (can't use wire wound resistors at RF)
resistor values, need to be more precision rated resistors, and of the
proper power ratings to dissipate the power you are trying to bleed off.
These resistors get expensive if you are trying to hold the drive RF level
to a "careful" tolerance.

Coax already is an attenuator (yes, we normally look for the least amount of
attenuation with feedline) and it already comes impedance "matched" for 50
or 75 ohm input and output impedance applications !!

 I have learned that the coax coil attenuator is the most reliable and
"Failsafe" solution and has the least amount of components to fail with
catastrophic results. It is also the least expensive to implement when you
consider the cost of YOUR time.  A small 75' spool of RG-174 is only about 2
by 3 inches in size and I have seen large spools of it at hamfests and
electronics surplus houses for 10 cents a foot or less. 

There are no active components with a coax attenuator that can fail and dump
too much RF into your transverter. If the resistor value in the lower leg of
the "T" configuration resistor attenuator "opens" then you instantaneously
dump almost full applied power into the device......POOF !! There goes that
transverter for the remainder of the contest......If the coax coil
attenuator goes "open", you simply lose drive to the transverter.

And, in Rick's scenario.....bleeding off 10 watts of RF is still 10 watts of
power that has to be dissipated in the form of heat. Inside of small
cabinets, this resistor attenuator heat can be significant if you run
lengthy periods of CW "CQing". The variance of the attenuator heat inside a
cabinet can even cause your mixer crystals to drift and then your dial
calibration starts to drift. So if you chose the resistor attenuator
approach, build in in a separate "Bud" box. 

Ron Rogers
-WB8ERB-
W9ICE Contest Group


-----Original Message-----
From: John Hoaglun [mailto:john at hoaglun.com]
Sent: Tuesday, September 02, 2003 12:01 PM
To: vhfcontesting at contesting.com; Rick Stoneking
Subject: Re: [VHFcontesting] Transmit Attenuator


Another suggestion....

Build an attenuator.  I built one for my 222 transverter and it is behind
the TR relay so that it doesn't effect the receive audio level.  I doesn't
make sense to use a high performance transverter only to knock down the
audio levels.

I ordered my resistors from RF parts.  It took me about 10 minutes to build
and test the circuit. Another 10 to add the perf board inside of the
transverter case and get the leads soldered in.  I think that my circuit was
Pi design... I got the info from W0JT.... he built me a spreadsheet to
calculate the values.

I have never been a fan of having extra coax lying around.

GL & 73

John Hoaglun - MCSE
KC0LBT - EN35
http://www.hoaglun.com

> > -----Original Message-----
> > From: Rick Stoneking [mailto:w2rds at arrl.net]
> > Sent: Tuesday, September 02, 2003 10:13 AM
> > To: vhfcontesting at contesting.com
> > Subject: [VHFcontesting] Transmit Attenuator
> >
> >
> > I need to attenuate a 12W output (144MHz) down to 2W to drive a 903MHz
> > transverter.  The radio's ALC circuit does not appear to be working so I
> am
> > trying to find out if there is a something I can buy or build to
attenuate
> > the signal.  Does anyone have any experience/suggestions?
> >
> > Thanks,
> > Rick
> > W2RDS

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