Re: {Collins} R-388 Spurious Response Problem




----- Original Message ----- From: "Robert Thomas" <ve3tou@xxxxxxxxxxxxxxxxxxx>
To: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
Cc: <collins@xxxxxxxxxxxxx>
Sent: Wednesday, July 28, 2010 3:55 PM
Subject: Re: {Collins} R-388 Spurious Response Problem


It sounds as if the signal is leaking into the first (tunable) IF, which tunes 2.5-1.5 MHz on some bands and 3.5-2.5 on others. (alternating between even and odd). If so, it would appear at 4.420, 6.420, etc. But I'm not quite sure what you mean by "center" of a band in this case.

Since there's no such thing as "infinite" feedthrough rejection, it may well be "normal", and if it still appears with all covers and shields in place, one fix would be a trap of some sort in the feedline. But first, if you're using coaxial feedline, make sure the shield is properly connected to the connector shell and is continuous throughout its length. Same with the line from the antenna tuner to the radio. Otherwise, the feedline acts like an antenna.

You don't say what kind of antenna you're using -- if you use, say, a 20 meter dipole, it'll knock the level away down, because antennas that are much smaller than 1/2 wave become very inefficient (and you want it to be inefficient at 1580 kHz).

I'd expect an antenna tuner to attenuate frequencies other than that for which it is tuned. This is definitely normal.

73
Bob
I think you have it. The signal is on all bands with even megacycles in the center of the dial, i.e., 4,6,8,10, etc. and not on those with odd center frequencies, i.e., 5,7,9, etc. Since the station is on 1580 its right near one end of the 2.5 to 1.5 IF. Its position stays exactly the same on all bands. All the covers are in place but the bottom is not screwed down, I rather think that would not make a difference. Right now the antenna is very makeshift, just a length of wire connected to an MFJ tuner going to the RX _ not very well shielded and plenty of place for signal to leak in. Anyway, a trap or a filter seems to be a reasonable cure. I just wanted to make sure there was nothing wrong, evidently there isn't.

--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx






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