The AM Forum

Hello Everyone
MOP radio gets closer and closer. Married life takes all those many free hours away. Now, there are a few free hours.
 
Question: The new MOP station will be running no iron, SSB linear / FT101 Exciter nice audio, etc.
The rigs have their own T/R relays. I want to have my R-390 on the receive side to hear the big scrote stations booming on the nice monitor system. No crappy little 3 inch speaker sound, please. When there's a lot of competition on 40M I can flip to the TS-440 and use the magic of I.F. filtering/shift, etc.

I picture back-to-back T/R relays.
The TX ports connected to each other and the RX ports connected together. The first relay common connection will connect to the antenna system. The "second relay" common point will connect to the linear. Both relays will be keyed at the same time. The purpose for this is to give me a point where there is a receive "tap" for the R-390. The JS side of this will be that the receive side will be terminated with two receivers. This doesn't sound very professional. There will be an external PTT to key the linear and the back-to-back T/R relays mentioned above.

What do you folks think?

Sounds like it could work but maybe I need an antenna coupler to isolate the receivers from each other.
Thanks and G'day

Fred  KC4MOP

  we handle thay problem where i work with receiver multicouplers. basically a broadband preamp to make up for loss, feeding  hybrid splitters. they are a bit pricey, but work well. recently i had a job that called for a multiple scanner installation at an EOC and found some nice multicouplers made for the SWL
marekt at a fraction of the price of the "pro two way" units. still expensive though. for about twenty years or so i have used plain old cable TV splitters- you know- couple two or three sets to one cable. they are true hybrid splitters, usually rated 5-500 or so MHZ, but work well down to 160 meters. you lose 3-5 DB per port, but at lower freqs like 75 meters where your effective sensitivity is noise level limited you never hear the difference. even on 10 i barely hear the loss. this allows me to run two or three receivers at the same time and pick the best one for the conditons at hand. to be efective at HF you need one hell of a stout preamp to make up for the loss, and i never found one that could stand the level of RF on my antennas, since i'm located about three miles from two 50KW AM BC stations. 

You can build a 1:2 hybrid splitter for about 2 bucks. 1/2 inch core a little wire and a noninductive resistor. Check the handbook. I bought some minicircuits splitters off epay cheap from KMEC. You won't need an amplifier
on the low bands. Handbook has some nice high dynamic range amps using 2N5109s or 2N3866. Or spend big bucks. and buy a multicoupler.

Thanks guys,
There must be a technical gap forming in my head since I have retired. But thanks to the resources here, I'll get revitilized. The CATV splitters and ARRL handbook sounds really good to me. With my RF sig gen I can see what losses are there.
Thanks
Fred  73 ............waiting for radial wire and trap construction for the mighty "L"

Fred,
I use a 1:4 splitter in line all the time without an amplifier for 75 and 160M
It takes 3- 1:2 splitters to make a 1:4.

With two narrow band receivers tuned to the same frequency where atmospheric noise predominates- I would just use a Tee-connector and short cables to each receiver.

Regards,
BobbyT

I find using a Tee makes it hard to compare performance between two receivers based on the input Z. Yup it will sure work though and do that sometimes. The splitter provides isolation between two RX ports. My homebrew one is about 28 dB.

Not too good an idea to use a tee on 2 receivers on the same frequency.   The front end filters might suck out the signal.  If you had one RX on say 75 M and the other on 20 M this wouldn'tbe a problem though.

    when i tried a T fitting to use two receivers on one antenna i found i could tune/detune one receiver's front end by simply moving the tuning or preselector on the other. and as mentioned above, setting one on 75 and the other on 20 can lead to big losses. preamps- even good ones from my job allways had too much gain and led to BC band intermod, etc. the theoretical loss of these splitters is only abot 3DB per port. only time i ever saw major loss was with four port cable tv splitters, and even then only at 15 or 10 meters.

Good points all around on using the Tee-connector method.
This was intended as a KISS (cheap) approach for two rcvrs on the same freq. Easiest way to check for interaction is to simply disconnect one or the other.
As Frank mentioned, if you have or can build an isolated splitter that's obviously the best approach. I keep a number of Merrimac PD-20-50 and PD-20-500 splitters around for just that purpose. As Chris noted, even the VHF types won't fall entirely apart down on 75m.
Regards,
BobbyT

Maybe I missed it, but why can't you use the receiver output (jack labeled something like Aux RX or similar on the the back of the 101, it's near the lower left corner), if you just want to put the R-390 into the receive RF chain? You can mute it in any number of ways. I've done this on 101 and 102 with no problemo. And you can hang more than one receiver off this port via a splitter.

You could use a simple "T" connector with 3.5 dB of loss if each receiver were 50 Ohms and broadband. Things get compliicataed when one or both receivers have either a preselector or a high pass filter between the antenna jack and the first mixer.  This may not be a problem if both receivers are on the same band.  If on different bands, and the receivers are different, you will have to experiment which reveiver to put on the lower band.

There is also the effect of whatever, if any, antenna tuner you have in your system. Mine acts like a high pass filter, and if set up for 75, severely attenuates 160. 

Lastly, there is the potential problems of birdies escaping one receiver and getting into the other. Of course, this can, and does happen even if the rigs are completely disconnected. For example, my R4-C puts out a bird that I pick up, though faintly, on my HT.

The better solution is the hybrid coupler, as suggested above. To test the isolation of the coupler, tune in a stable fixed level signal on one receiver and test the effect of making the other hybrib porteither  open, shorted, or terminated with a 50 Ohm load. If so equipped, turn on the calibrator in one rig and compare its level in the other using a T and then when using a coupler.