Well I did some testing this evening with some cables, several different lengths. The original is a 4' length of Superflex FSJ1, power out of the duplexer is 115 Watts, now tested on a Bird, and a small tweak on the duplexers, so I am very close. I changed to a 5' cable, 90 watts, 6' gave 90 watts, a 4.5' and got 100 watts, 3.5' gave me 110 watts, so I guess my magical number was 4'. I guess they say, if it ain't broke, don't fix it. "And no, I did not tune the duplexers while transmitting".
Thanks to all whom replied, I think I am as close as I am going to get, and the system seems to be working like it should, other than the normal receiver problems that I have, and they will live that way until the receiver is changed out.
Mathew
Kevin Custer <[EMAIL PROTECTED]> wrote:
Mathew, et al,
Without very expensive test equipment, there is no way to "calculate"
the correct length needed to match a transmitter to a duplexer. The
correct length will seldom be some an exact wavelength or simple
fraction of it. The terminal impedance of a transmitter can change with
the change of output power level, so the correct length at one power
*could* be different from another outputted power. For most of us, this
length must be determined experimentally, and is another reason why a T
Matcher is nice, because it's simpler to tune it, then cut cables.
In the case of Charles' repeater, I believe he was simply lucky when he
cut the cable to length, and all was well.
Kevin
Mathew Quaife wrote:
> Seems like I might be learning something here. I took for assumption
> that the shortest cable length to from the transmitter to the amp, and
> from the amp to the duplexer was the way to go. Am I to understand
> that a certain length should be used, ie a full wave length perhaps?
> I have some cables lying there, I could try them and see if I notice
> any changes. I know 20 watts will not make the difference, but am
> rather more concerned with spurs that might make it's way out of the
> repeater.
>
> Mathew
>
>
> */Charles Miller <[EMAIL PROTECTED]>/* wrote:
>
> Mathew,
>
> I would think that you have a little more insertion loss on the TX
> side than
> you think, or the cavity may need a little tuning. You may also
> need to try
> different lengths of coax between the transmitter and duplexer.
>
> I have had to change the coax length on one of my UHF repeater
> because the
> coax was to short. After calculating the correct length for the
> coax I was
> using for a full wavelength I was getting the proper RF out.
>
> I believe someone else suggested doing this, and it is a very good
> idea. If
> all else fails, and this is NOT recommended with 200 watts RF
> input, you can
> make very small adjustments on the tuning rods. DO NOT tune the
> rods with
> the transmitter on as you can ARC the cavity tuning slug and
> damage it. Try
> 1/8th turns at first and see if the output power goes up or down.
>
> I have done this in the past, however it is NOT recommended. Great
> care must
> be taken when tuning a cavity in this manor as the cavity can be
> damaged
> easily at this power level. If you have 25 watts available you
> should be
> able to tune it easier but still be careful.
>
> Charles Miller
>
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