The main effects of coax choice for the stubs:
1. voltage handling capacity -- The stub terminates in open/short and
thus the VSWR will be very high on other than the design fundamental
(pass) frequency; e.g., harmonics. Fortunately, the power levels should
not be high at these frequencies. Nevertheless, if transmitting at 3850
kHz on a stub cut for 3525 kHz, the SWR on the stub will not be ideal
and some power will be present at less than ideal frequencies.
2. loss characteristics -- Low-loss coax (e.g., hardline) will have a
sharper rejection notch than higher loss coax: deeper null but narrower
in bandwidth.
-- Eric K3NA
on 07 Jul 17 21:44 jeremy-ca said the following:
> When I was building the 2 station operation here I tested a wide variety of
> flexible coax looking for the best null. CATV RG-11 was the best, giving a
> solid 25dB+ null, The impedence doesnt matter. I would suspect the the LMR
> would be similar in performance.
>
> A very easy way to tune the stub is to cut it a bit long, hook it to your
> receiver thru a T connector and with a noise bridge simulating the antenna.
> Spin the RX dial back and forth and note the frequency of the deepest null.
> Then trim until you have it centered exactly where you want it. On 80/160
> you could use vacant band noise in the middle of the day.
>
> Carl
> KM1H
>
>
> ----- Original Message -----
> From: "Robert Pack, NX5M" <nx5m@txcyber.com>
> To: <towertalk@contesting.com>
> Sent: Tuesday, July 17, 2007 9:18 PM
> Subject: [TowerTalk] LMR240 for stub filters?
>
>
>
>> Wondering what the group thinks about the use of LMR240 as stub filters
>> for the 80 and 160 stations here. I have a bunch of it that I am not
>> going to use for anything so if you guys think it would work for this
>> purpose at least I can use it for something constructive.
>> This stuff would be used behind 1500w amps. I know that someone has
>> melted down stubs made of RG58 which is pretty much the same as LMR195. I
>> believe LMR240 has a higher power rating than RG58 (which I would never
>> use anyway) so this got me to thinking that 240 might work.
>> What do you guys think?
>>
>> 73,
>> Bob NX5M
>> _______________________________________________
>>
>>
>>
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>>
>
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