OK Al, I am using 75 ohm split on two Micors with a preselecter on each.
Seems to work rather well. I bought a CATV 10 db adjustable gain splitter
from Radio Shack several years back and converted it to 12 volt to run on
battery back-up, but never had the need to use it till now. Sounds like
we're along the same thinking on this.
So we take the preselecters off the Micors, put on the RS 4-way splitter,
gain set to over come loss. What about jumpers, 75 or 50 ohm ? Does it
really matter, and do they need to be 1/4 wave length of the freq. they are
hooked to ? Thanks, Bruce KD4BOH.
----- Original Message -----
From: "Al Wolfe" <[EMAIL PROTECTED]>
To: <Repeater-Builder@yahoogroups.com>
Sent: Thursday, August 04, 2005 12:56 AM
Subject: [Repeater-Builder] Re: receiver combining (Was Digest Number 3263)
>
>> At 8/3/2005 06:14 AM, you wrote:
>>>Bruce,
>>> One common method of combining two receivers is to use a coax tee at
>>> the
>>>antenna feedline and 1/4 wave pieces of 75 ohm coax from the tee to each
>>>receiver. This can be expanded to four receivers using six pieces of 75
>>>ohm
>>>coax, but probably isn't worth the effort.
>>
>> But what if the RXs have high Q filters in the front-end that look like
>> an
>> open at the other RX's freq.? The 1/4 wave transformer will turn that
>> open
>> into a short at the T, shorting out the RF that otherwise would go to the
>> other RX.
>>
>> Sounds unlikely? I know of one group that did this to their repeater a
>> while back. The result was a deaf repeater providing a disservice to the
>> amateur community for many years.
>>
>> Use a power splitter, either CATV or 50 ohm (Mini Circuits). The
>> port-to-port isolation will save you much grief in the long run.
>>
>> Bob NO6B
>>
>
>
> Bob,
> If it was the truly the case that the receivers had a high impedance
> at
> off channel frequencies then you could just hook them all together in
> parallel and be done with it. However, most receivers have very low
> impedances off-channel. (Also, few front ends are ever actually 50 ohms
> resistive anywhere for that matter.) Because of the low impedance of
> off-frequency front ends, receivers with noise blankers usually have their
> two receiver inputs in series and the blanker receiver must be tuned
> several
> mhz. away from the main channel to not cause the sensitivity to be
> compromised.
>
> I suspect that if the group with the deaf repeater had added a few
> inches of line to the other user's receiver their problem would have been
> solved.
>
> I have aircraft receivers from the FAA with two antenna jacks, one in
> and one out. Several receivers can be daisy chained together this way. The
> last receiver has a shorted "N" connector on the antenna out port.
> Effectively, all the different receivers' first RF coils are in series
> with
> each other.
>
> The 1/4 wave, 75 ohm coaxes were/are commonly used with DFE (dual front
> end) receivers or stations with two receivers.
>
> As you say, however, the splitter is usually a superior method of
> combining receivers from one feed. The ones from MiniCircuits provide 20
> db
> or more isolation between ports and the lengths of feedline from the
> splitter to the receiver isn't critical. The complex reactance's of the
> receiver front ends are isolated from each other. A dead short across one
> of
> the output ports will have minimal effect on the other ports.
>
> Another benefit of the splitter and its isolation came to bear in a
> situation I had several years ago where we had several UHF receivers
> sharing
> a common antenna using the 1/4 line technique. Very strong signals on one
> receiver could often be heard on another receiver in the cluster. Turned
> out
> that some of the mixer injection from one receiver was getting back out
> though the RF stage and into another receiver. A splitter with 25 db of
> isolation between ports got rid of the problem.
>
> 73,
> Al, K9SI
>
>
>
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>
>
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