On 06/12/11 12:06 AM, Mike Tubby wrote:
> On 08/06/2011 16:22, Al Kozakiewicz wrote:
>> Maybe not quite amps related, but this is the best forum I know of for RF
>> expertise ...
>>
>> Without belaboring irrelevant details, I'm calibrating an LP-100A wattmeter
>> I just built. With a brand new MFJ 300W dummy load I measure 50.1 ohms with
>> my old Fluke DMM. The dummy load is connected to the coupler with about 6
>> feet of RG-8U. It appears as though the resistance of the dummy load
>> increases with frequency, to the point where it is 65 ohms in the 20m band
>> and the LP-100 can no longer compensate.
>>
>> After fiddling with the coupler with no joy I decide to connect my MFJ-259
>> to the cable and verify that the resistance is actually 50 ohms across all
>> the HF bands. Turns out that it's not. In the 20m band the impedance at
>> the end of the RG-8 has risen to 66+6j.
>>
>> This is as simple a setup as I can imagine - a 50 ohm resistor in a metal
>> box and 6 feet of new 50 ohm coax cable. Tonight I will test again without
>> the cable if I can find a M-M UHF adapter. But I'm puzzled as to what the
>> issue could be.
>>
>> Any ideas?
>>
>> TIA
>>
>> Al
>> AB2ZY
>
> If its MFJ its likely to have been built down to a price and have a
> significant reactive component.
I think anything from MFJ is pretty poor. A friend describes it as "More
****ing
Junk".
> Several thoughts:
>
> 1. do an A-B comparison with a *real* dummy load, for example a Bird
> 8080 for low power, Bird 8201 for 500W or even a Bird 8251 for 1000W
> (for what it's worth there's someone in California selling new Bird
> 8251s on Ebay for sensible money)
<snip>
> Mike G8TIC
I'm fairly lucky in having a 5 kW Bird load on semi-permanent loan. But when I
was active on 2m (long ago), my dummy load used to consist of a long length of
RG 58 coax. That was good to 1 kW for a few minutes, though I must admit I
melted it on more than one occasion.
If you can get a length of coax able to attenuate a signal by 10 dB (not hard
at
high frequencies, more difficult at low frequencies), then the reflected signal
will be 20 dB down, as it will suffer a 10 dB loss getting to the end of the
coax, and a further 10 dB getting from the end back to the transmitter. That's
a
VSWR of 1.22:1 - good enough for practical purposes.
The first part of the coax can't be coiled up, as that dissipates most of the
heat. But further down, it can be coiled up, as the heat dissipated is much
lower.
I'm currently using a 250 W 10 dB attenuator as an HF dummy load. I just leave
the end open, even though I know any sort of dummy load (no matter how poor),
attached to the end would improve the VSWR.
I can't be bothered to use the 5 kW Bird load, again due to the size and
weight.
A 250 W 10 dB attenuator is much easier to move around.
Many people have lying around a long length of coax. It makes a pretty decent
dummy load.
Dave G8WRB
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