I have a rather oddball 2500W Bird built for RCA for the old 900MHz cell
band, return loss is still under 22 dB at 1296. The snout has a long taper
from an EIA flange to type N and the coupler is built into the snout with a
remote meter that is only scaled to 1500W. Its long and ungainly so I have
it stashed away under the basement stairs and use it just as a load with a
43 on the work bench.
I suspect that MFJ could benefit from a small low value variable cap to
cancel the XL, that was what I used ages ago to a Heath Cantenna to get it
to work on 2M.
Carl
KM1H
----- Original Message -----
From: "Mike Tubby" <mike@tubby.org>
To: "Al Kozakiewicz" <akozak@hourglass.com>
Cc: <amps@contesting.com>
Sent: Saturday, June 11, 2011 7:06 PM
Subject: Re: [Amps] Dummy load varying R
> 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.
>
> 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)
>
> 2. change the length of coax and see how the impedance moves round the
> smith chart... if you're careful with your selection of length(s) you
> can probably find a point that hides the problem
>
> 3. *real* dummy loads have a single large resistor element surrounded by
> a double metal cone that is tapered to create a constant impedance
> taking in to account the length of the resistor - the screen has holes
> or slots in it to allow the oil to circulate
>
> 4. if/when you decide that your MFJ isn't all that you hoped it would
> be, either build a load or buy a good one...
>
>
> Mike G8TIC
>
> PS. While Bird officially state that their dummy loads VSWR at 1.1-1.5 I
> have never had a return loss of worse than 30dB for all frequencies up
> to and including 2m
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 10.0.1382 / Virus Database: 1513/3696 - Release Date: 06/11/11
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|