[ICOM] IC756ProII Receive Antenna
JohnD
wm7a at cox.net
Wed Jan 12 16:31:26 EST 2005
Jerry,
The answer is yes. The 756 series (756, PROII and PROIII) will toggle
between the Tx antenna and the Rx antenna when set for ANT "1/R".
But, as you have guessed, the radio does not disconnect the rx antenna when
transmitting. It does, however, have some protection circuitry.
That said, I had problems with my friends PROII and my new PROIII when
transmitting on ANT1 and receiving on on the Rx Antenna port (ANT "1/R").
When transmitting on 160 or 80, I tried to tune the antenna and could not
get the SWR down below about 3:1. I lowered the power and tried tuning and
all was OK. As I gradually raised the power the displayed SWR went up
significantly at anything over 20W transmit power. I thought maybe
something had gone wrong with the antenna, but could find nothing. I
switched to my old 756 (classic) and all was OK.
After some experimentation, I found out that if I deselected or unhooked the
Rx antenna (K9AY loop), all was well again. Well sort off. I could only
run the linear if I disconnected the loop completely.
What I found was that the Tx antenna was coupling into the Rx antenna to the
point that there was so much power coming into the PROIII on the Rx antenna
port that the radio was shutting down. Unfortunately my Tx antenna is only
10 feet away from the Rx antenna at its nearest point. I measured about 25V
peak to peak coming from the loop across a 50 ohm load when only
transmitting at 100W. No wonder I am having problems. It turns out that the
Rx antenna protection circuit in my old 756 is quite a bit different from
that in the PROIII (and PROII ), so I did not notice the problem before. To
fix the problem, I installed a relay on the Rx port of the PROIII to
disconnect the loop and short the radio's Rx Antenna port to ground when
transmitting. You can see this arrangement in the interface electronics for
my K9AY loop at http://members.cox.net/wm7a/Images/Control%20schematic.jpg
John DeRuiter, WM7A
http://members.cox.net/wm7a/
-----Original Message-----
From: icom-bounces at mailman.qth.net
[mailto:icom-bounces at mailman.qth.net]On Behalf Of Jerry Keller - K3BZ
Sent: Wednesday, January 12, 2005 12:06 PM
To: (Reflector) ICOM; (Reflector) ICOM Yahoo
Subject: [ICOM] IC756ProII Receive Antenna
When I select the Receive Only antenna "Ant 1/R" (by depressing and holding
the Ant 1/Ant 2 button)
will the ProII then toggle between "Transmit on Ant 1" and "Receive on the
Receive Only antenna" ?
I would test this, but I'm concerned that a beverage antenna connected to
the Rx Antenna jack might
remain connected during transmit on Ant 1, with adverse consequences to the
radio. I'm running a KW
from a PW-1 amp. Should I be concerned about this? Are protective measures
needed?
73, Jerry K3BZ
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