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.
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.
----- Original Message -----
From: "Ken Brown" <firstname.lastname@example.org>
To: "Discussion of Ten-Tec Equipment" <email@example.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
>> 50 ohms in and 50 ohms out.
>> If you need less loss then here values for 6 dB. Input series R is 17
>> made of 4 x 68 ohm 2 W carbon resistors in parallel. The shunt R is 67
>> made of 4 x 270 ohm 2 W carbon resistors in parallel. the series output R
>> a 2 watt 18 ohm resistor. Again, some 6 dB attenuation with about 50
>> in and out.
>> These values will handle 25 to 35 watts of SSB with no problems. Watch
>> 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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