[ICOM] is there some sort of protector on the RX ANT, input of a 756 Pro?

[Maciej Wieczorek]

Hello,

I have an IC 775.

Grounding RX antennas (as well as RX port in the transceiver) is not always 
the best idea, I prefer to load RX antenna with 50ohm / 5-10W resistor, 
during transmitting.

By the way, I mesasured 2-3watts back in the shack on RX antenna output, 
when putting 1,2 KW into 80m horizontal loop which is very close (about 
8-10m) to K9AY receiving array. Simply connect some power meter and you'll 
see how much RF picks your RX antenna and when. In my case 5 watt resistor 
solved the problem.

73's
Mac SP2XF
----- Original Message ----- 
From: "Diane and Edward Swynar" <deswynar at xplornet.ca>
To: "ICOM Reflector" <icom at mailman.qth.net>
Sent: Wednesday, February 09, 2011 8:29 PM
Subject: Re: [ICOM] is there some sort of protector on the RX ANT,input of a 
756 Pro?


> Hi Gary,
>
> I very quickly noted the absence of any "protection" of the receive 
> antenna
> port in my 751A the very day that I transmitted into a kilowatt amplifier,
> with a 1500' Beverage antenna connected to the port: the RF feedback 
> getting
> into the transceiver made for all kinds of "...weird & wonderful" scope
> patterns on my SB-610 monitor...!
>
> The solution, of course, was the incorporation of an external 12 VDC relay
> that automatically grounded the receive antenna port when 
> transmitting---the
> control voltage was secured directly from the multi-pin jack at the back 
> of
> the set.
>
> I have no idea why Icom never did this "...de riguer", at the 
> factory---but
> they didn't, so one is forced to improvise in the field! Hi Hi.
>
> ~73~ de Eddy VE3CUI - VE3XZ
>
>
> *****************************************
>
>
> ----- Original Message ----- 
> From: "Gary P. Fiber" <gfiber at comcast.net>
> To: "ICOM Reflector" <icom at mailman.qth.net>
> Sent: Wednesday, February 09, 2011 2:04 PM
> Subject: Re: [ICOM] is there some sort of protector on the RX ANT, input 
> of
> a 756 Pro?
>
>
>> I am not sure of the newest Icom products, but I can tell you the
>> earlier radios, IC-745, 751 / A, 761. 765, 781.... NONE of them had any
>> sort of protection in the receiver input.
>> If you removed the coaxial jumper and attached your receiving antenna to
>> the Receive Antenna In jack, you were connected directly to the receiver
>> front end.
>> Use of some sort of protection or relay circuit to remove the RX antenna
>> during transmit was always recommended at least by myself when I worked
>> at Icom.
>>
>> Using the UHF (S0-239) connector there is a small relay between that
>> connector and the Receive Antenna Out jack.
>> During transmit that relay would open placing an air gap between the
>> Transmitter output and the Receive antenna input so the receiver front
>> end would be protected.
>>
>> I was never sure what the reasoning was for the ability to open the
>> circuit to having an unprotected receiver input, maybe a preamp.
>> So the long and short of it, if you attach a fine receiving antenna to
>> the receive antenna input you need some sort of signal attenuator that
>> operates when you transmit.
>> Sending 100 or 1500 Watts out your transmit antenna will certainly cause
>> you likely high millivolts of RF energy presented to the receiver input
>> being picked up by your receiver antenna.
>>
>> Gary K8IZ.
>>
>> On 2/9/2011 9:56 AM, Carl N9GC wrote:
>> > The 756 line has long had a history of that sort of failure. Burghardt
>> > Electronics built a protection circuit for this purpose a number of
>> > years ago. They only repair rigs now but may still have the protection
>> > circuit available.
>> >
>> > 73, Carl N9GC
>> > ----
>> > Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
>> > Icom Users Net: Sundays, 1700Z, 14.316 MHz
>> > Icom FAQ: http://www.qsl.net/icom/
>> > To support QSL/QTH.net: http://www.qsl.net/donate.html
>> >
>>
>>
>> -- 
>> Gary P. Fiber K8IZ
>>
>> General Radio Telephone Operators License PG-19-6691,
>> with Shipboard Radar Endorsement.
>> Washington State Resident.
>>
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>> ----
>> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
>> Icom Users Net: Sundays, 1700Z, 14.316 MHz
>> Icom FAQ: http://www.qsl.net/icom/
>> To support QSL/QTH.net: http://www.qsl.net/donate.html
>
> ----
> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> Icom FAQ: http://www.qsl.net/icom/
> To support QSL/QTH.net: http://www.qsl.net/donate.html
>