The bandwidth comment is correct Eric. I had to swap stubs between CW/SSB
contests for 80/75M. Other bands were adequate set in the middle.
My initial attempt was CATV RG-59 which arced thru at 1200W or got very
warm.
I often thought about installing a second set of stubs 1/4 wave away as that
concept works very well at VHF/UHF when installing two relays to increase
isolation in mast mounted preamp setups. But I improved the phase noise
performance of the TS-940's so never followed thru.
Carl
KM1H
----- Original Message -----
From: "Eric Scace K3NA" <eric@k3na.org>
To: "jeremy-ca" <km1h@jeremy.mv.com>
Cc: "Robert Pack, NX5M" <nx5m@txcyber.com>; <towertalk@contesting.com>
Sent: Thursday, July 19, 2007 11:19 AM
Subject: Re: [TowerTalk] LMR240 for stub filters?
> 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
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk@contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>>
>>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|