Greetings,
Re: "having selected the separate receiving antenna, RF is fed back into the
[radio]"
If you notice RF feedback from an RX antenna as described (below, or with
ANY radio/RX-antenna combo), I can suggest one possible fix that's easy to
implement. This pertains to most ANY type of radio/receiving antenna, be it
a Beverage, EWE or, something like K9AY's receiving loop system (QST Sept.
1997).
(I think, but am not sure: The FT-100MP *might* have built-in protection
form RF feedback via the RX antenna port, by perhaps the FT-990 *might*
not??)
While the suggestion below is good, you can get away by spending a LOT less
money; just use a relay to send the RX antenna to ground during transmit...
For more details on this, see the articles on my web page, where you'll find
one titled "Beverage Notes," and, more to the point, my follow-up article,
"More Beverage Notes," where this exact problem (RF feedback from an RX
antenna) is discussed.
The best way to get into the subject might be to read both article--in
order--with "Beverage Notes" first, and then "More Beverage Notes." Again,
the RF feedback issue could apply to most any RX antenna, so it's not
Beverage antenna specific.
You'll find other Beverage-antenna-related information on the "Beverage
Info" page, too. Here's the URL's:
My main page: http://www.geocities.com/CapeCanaveral/1138/index.html
And the
Beverage Info page:
http://www.geocities.com/CapeCanaveral/1138/bev-page.html
Hopefully, you'll find something of interest there...
Enjoy!
73, Glenn Swanson, KB1GW
Avon, Connecticut
Home: kb1gw@snet.net
Work: kb1gw@arrl.org
========================
>>re:[Yaesu] FT990 Receiving Antenna
>>
>>Kok Chen (kchen@apple.com)
>>Tue, 4 Nov 97 09:50:08 -0800
>>
>>During transmitting and having selected the separate receiving antenna,
>>RF is fed back into the 990 which can lead so far, that the internal
>>power-supply or the tuner oscillates and the supply voltage is breaking
down,
>>which can be seen because the S-meter lamp becomes dark and darker ...
>
>If you look at the diagram on page 27 of the manual (in the
>side-bar titled "Receive-Only Antennas and Separate Receivers"),
>it seems to bear your observation out. The TX/RX switch is not
>where I would put it for connecting a receive antenna all the
>time.
>
>However, there is a really nice use for the FT-990 "receive
>antenna" connection if you are using a JPS noise canceller
>(I think MFJ makes one too, and CQ Magazine published the
>schematics for one recently -- I have no idea how that one
>gets phase reversal, but the author swears it works).
>
>If you were to connect the RX Antenna Out of the FT-990 to the
>Antenna connector of the JPS, and connect the RX Antenna In of
>the FT-990 to the Tranceiver connector of the JPS, you get
>three things:
>
>1) the TX/RX switch of the FT-990 is used instead of the TX/RX
> relay in the JPS (much better switch over transients).
>
>2) RF power does not get into the JPS (I don't use a linear
> amp, but this should do the trick too),
>
>3) you can enable and disable the noise processing by just
> switching the RX ANT switch on and off, respectively.
>
>73
>
>Kok Chen, AA6TY
>
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