Re: [TenTec] Home brew RF power attenuators

["Bob McGraw - K4TAX" <RMcGraw@Blomand.net>]

Fair Radio Sales for some of them.   Mouser for some and Surplus Sales of 
Nebraska.  Although the latter is a bit pricey but they do have the parts 
and values often needed.  They also have some non-inductive values of 
slightly higher wattage.  I've found these to perform good up to about 30 
MHz or so.  Probably best to check with a bridge to see if any resonance 
exists near the intended operating frequency.
http://www.surplussales.com/Resistors/CarbonComp/Res-CarbonComp_5.htm

In some cases I use the Pi configuration.  I find easier to mount components 
thus being two BNC chassis connectors on a small metal plate, a resistor 
from each center pin to ground and a resistor between the two center pins.
73
Bob, K4TAX

----- Original Message ----- 
From: "Ken Brown" <ken.d.brown@hawaiiantel.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Monday, December 31, 2007 9:48 PM
Subject: [TenTec] Home brew RF power attenuators


> I also built an attenuator from 2 watt carbon composition resistors. My
> application was a Kenwood TS-440 driving a SGC SG-500 amplifier. The ALC
> circuitry in the TS-440 made the keying envelope funny at anything much
> below 90 watts output. The SG-500 only wanted 50 or 60 watts drive, and
> anything higher would kick in the built in input attenuator, which would
> no doubt also make for a weird CW envelope. The attenuator I made would
> also only handle about 25 watts continuous, so I built it in a one quart
> paint can and filled it with mineral oil. A sort of "cantenuator".
>
> There are web sites with attenuator calculators, wherein you enter
> impedance, dB loss and whether you want a pi or tee configuration. I
> have generally found tees easier to get the right resistor values. That
> may not be the case for all attenuation values though.
>
> Bob, where do you get your two watt carbon composition resistors?
>
> DE N6KB
> > The 10 dB attenuator is a T configuration.  The input series R is 25 ohms
> > made of 4 x 100 ohm 2 W carbon resistors in parallel.  The shunt R is 33
> > ohms made of 3 x 100 ohm 2 W carbon resistors in parallel and the series
> > output R is a 2 watt 27 ohm resistor.  Close enough to 10 dB attenuation 
> > and
> > 50 ohms in and 50 ohms out.
> >
> > If you need less loss then here values for 6 dB.  Input series R is 17 
> > ohms
> > made of 4 x 68 ohm 2 W carbon resistors in parallel.  The shunt R is 67 
> > ohms
> > made of 4 x 270 ohm 2 W carbon resistors in parallel. the series output R 
> > is
> > a 2 watt 18 ohm resistor.  Again, some 6 dB attenuation with about 50 
> > ohms
> > in and out.
> >
> > These values will handle 25 to 35 watts of SSB with no problems.  Watch 
> > CW
> > key down or tuning times as the dissipation of the R's may be exceeded.
> >
> > If you need other values of loss or input or output Z, let me know.  I
> > figure the values for you.
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