On 2/11/2014 3:36 PM, George Thornton wrote: >>> I have a dual receive K3. If I have both receivers going, one on each >>> channel, and I broadcast on one of these antennas, am I in any danger of >>> overloading and frying the other receiver?
Even at QRP - you could have a problem. I recently put up a receiving loop for 160 in a small backyard at about 25 feet from and 25 feet below from an inv vee. I wanted to take a quick look and was surprised that at 10 watts out of the K3 , it would start activating the protective relay for the sub receiver. Personally I would NEVER rely on the relay to protect the sub rx long term . As far as I am concerned it is just there to tell you you should do something different NOW. . I have a VNA2180 anatyzer and can measure the coupling and find the coupling between the loop , and the inverted vee is only down 27 dB worst case coupling . 27 dB down is a power ratio of 500 so 10 watts was putting about 20 milliwatts on the sub rx input. For a frame of reference (5mw is S9 +80dB or 500millivolts , +7 dBm so 20 mw is S9+86 dB - or 1 volt , +13 dBm .) In addition to the inv vee at 39 feet I have a KT34 at 42 feet , a rotary dipole at 50 feet at right angles to the KT34 , a 30 m and 12m dipoles hanging from the tower. There are MANY combinations in that mess that are only down about 30 dB in worst case orientations and frequencies. Since you may not have an analyzer or a freind who could make the measurement for you , I thought of one thing you might be able to do. If you have an external QRP wattmeter , just feed a known power into one antenna and put the SWR bridge on the "coupled" antenna - with the coax to the normal input and put a 50 ohm dummy load - or even just a 50 ohm resistor on the output of the bridge and you can read the coupled power. As someone else suggested , an external protective device uisng a DPDT relay (like Kestler KS2E-M-DC12 which has 2 amp contacts and better yet gold flashed contacts for "dry" circuits") only $1.75 at All Electronics and a GP PNP and couple resistors , couple bypass caps , a reverse diode across the relay coil and a box should cost less than $5 for a lot of security - driven directly off the KEY OUT from the K3.) I like the DPDT because it allows you to both short the rx input and open the line to the antenna . That gave well over 100 dB of isolation at 160 thru 40 - I was not interested above that so did not look at it but I would expect 80 - 90 dB easliy at 30. There are also rx proetctive device using diodes which do not involve relays - Array Solutions AS-RXPEP is one that is clever , using step up and step down transformers with the diodes between - theyclaim it will handle 10 watts ICAS on inoput and lints output to o dBm pr about 223 mv. I am sure ther are many other designs around, I personally do not want to worry about diodes hnaging across a rx input . 73 Hank K7HP ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
