[Icom] Re: Topband: YOU NEED AN RX PORT SWITCH CIRCUIT!

Fri, 01 Nov 2002 07:44:16 -0500

The RX input (port) is "ACTIVE" during transmit.  It's like a PA system with
the gain up too high...screaming feedback but you cant hear it since it's
RF.  Makes the rig and amplifier go crazy and self-destruct the front-end of
your radio.  Many have the problem but too few recognize it.  The solution
is the receive antenna (or beverage etc) MUST be disconnected and/or
grounded during transmit.  Most folks realize this after having their rig
repaired a couple times...de gary, kd9sv

----- Original Message -----
From: "BILL MCHUGH" <[email protected]>
To: <[email protected]>
Sent: Monday, October 28, 2002 9:32 AM
Subject: Topband: YOU NEED AN RX PORT SWITCH CIRCUIT!

> There have been several posts about the need for a proper RX-only antenna
> switching/grounding circuit.  Most have dealt with lightning protection, a
> few with interaction between the RX port and the radio/amplifier. Here's
> some proof of the need for such a switch.
>
> I had a Beverage ant here on the farm whose fed end came to within about
20
> feet of one set of guys for my vertical.  Whenever I selected that
antenna,
> and while using my FT990/Clipperton amp on topband, the amp would suddenly
> break into oscillation and pull a lot of current.  I eventually moved that
> Beverage antenna to another location and that solved my problem.  I
thought
> that was the end of it.
>
> Well, I have started to use the 990 with an AL80 at our summer home, and
> last weekend I went to tune up for the contest and WHAM!...the amp broke
> into oscillation!  Again, I was using a Beverage whose end is located
fairly
> close (20 feet) to guys on my tower.
>
> I discovered that the switching cable to my Bev disconnect/TX switch
somehow
> got pulled out and the Bev was not being switched to ground, and the RX
port
> of the 990 was was not being grounded in TX (I do not switch the linear
> through this circuit...I plan to now!).
>
> So the problem is obviously the 990 switch board; while a small PCB relay
is
> used to switch the antenna connection, there is obviously considerable
> bleed-through to the RX port, creating feedback which neither amp seems to
> like!  I don't know exactly what is happening here, but when both the RX
> port and the Bev are grounded in TX, the problem disappears.
>
> I suspect that this problem resides with several other radios besides the
> 990.  I also wonder what kind of garbage is being radiated on TX when
> running barefoot in this condition when everything seems to be OK!
>
> So, if you don't have a RX port switching/grounding circuit on your
> transceiver, it is time to get building!  (By the way, KD9SV's excellent
> circuit in QST a couple of years ago is now available commercially built
and
> tested.  While Gary's circuit uses reed relays, I employ open-frame units
> with wider spacing for a little more protection in case of a nearby touch
of
> Thor).
>
> Bill, VE3NH
>
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