I have a bunch of communications receivers and 4 outdoor antennas. I find myself switching the antenna connections all the time. A couple of weeks ago I spotted something on that auction place that got me thinking and I ordered one. It arrived today and I think it's going to work.

What is it? It's a pro quality switching matrix for switching RGB video. It has applications like classrooms and theaters and the like. The one I got has 8 inputs and 8 outputs. An "input" is actually 6 inputs consisting of 3 video channels, horz and vert sync channels, and two audio channels. The 8 outputs are also 6 outputs each. The matrix switch allows any input channel to drive any output channel. Since the outputs are buffered, one input can drive all of the outputs. The video channels on this particular unit have a 3db bandwidth of 450 Mhz and go down to near DC (it says DC I think but I'm not sure if that was for the video channels or not). They claim that crosstalk is -90db at 1mhz and gets worse as frequency goes up but is still pretty good all the way at the top (again, I don't recall the number up there).

To top it off, the switch has an RS232 port and decent software to configure the switch with the mouse. It also has front panel pushbuttons if you prefer manual configuration. I paid 100 bucks plus shipping and it worked when it got here today. I was worried that the digital stuff in the box would fill the radio waves with garbage but it looks pretty darned clean to me so far. I need more time to look everywhere but it is spotless down on the longwave band and on 160 meters. I couldn't see any rise in the noise floor when I tested these two bands with and without the switch. I would assume I would see some on higher frequencies.

So now I can plug 8 receivers into it and my 4 antennas and go to town. I am a little bit worried about static susceptibilty but my situation is not so bad that way. My two verticals are transformer coupled with the secondaries driving twin lead back to the shack. Because I use them at LF, the transformers are big step-downs. The transformer primaries have one end connected to the antenna element and the other to a ground rod at the base. This is not going to have static buildup issues and the step-down reduces the size of the spikes somewhat. I have preamps connected to them and have not blown anything yet. Then I have a short "longwire" of about 90 feet (it's short at LF) with a 9 to 1 balun. The connections are just like the vertical. Finally I have an active whip.

I'm pretty pumped about this setup and I hope it works out.

Tony

edit: oh, the switch is an Extron crosspoint Plus...

Great idea!

As long as you keep the leads short, you shouldn't have any impedance problems.

Oh well. It was an interesting idea but....

Too noisy. I'm not talking about spurs. Its really clean as far as that goes. But I have S3 broadband noise with nothing connected. When I looked at it the first time, I had my preamps in front of it on longwave. The preamps make the SNR look OK. But on shortwave, or anything without a preamp, it is not OK. The noise level is pretty flat, all the way up to 30MHz.

It's too bad. The switching setup is fantastic. I even ran a pair of cables over to the test bench so I could bring any antenna over there and send a signal generatpr back to any radio. I also learned that the audio switching can be independent of the video. I could route audio from the radios over to a nice amplifier/speaker.

I suppose I could build some more preamps, one per antenna....

In the back of my mind I was expecting something like this. I was just hoping it wouldn't be quite this bad.

Sniff...

Tony

PS, Isolation is also extremely good. I can listen to the local 50000 watt Broadcast station on the R-390a and then switch the antenna to an unconnected port and I hear NOTHING...except s3 hiss.

I think preamps are the answer, I am running WSPR on 20 meters right now (receive only of course) and I just logged a VK4, a VK7 and a ZL1 (2 Aussies and a Kiwi). All are running 5 watts. This is through the switch with a 10db preamp. I still get a few db rise in the background noise when I connect the antenna at 30 Mhz so the switch must be adding 7 or 8 db. Now to see if I can get a few hours of sleep.


Tony

it probably will "work" to a certain extent, BUT video is a lot more forgiving than the RF spectrum. For RF, you have signals like a MILLION times stronger than others, and you don't want the stronger signal to modulate the weaker one. You'd never notice such a thing going on in a video signal.

So you'll probably have all kinds of cross-mod and a constant background noise level from all the total cross-modding going on.

There are many on line examples of switching used for contesting and DXers. Ive built my own which allows using up to 12 Beverage receiving antennas (10 in use) that can be then switched to 6 radios in the basement (Two modern transceivers and the others vintage gear operating benches). Another switch allows transmit or receive antennas to be used and then there is another switch to select transmit antennas!

Sounds involved but easy to use in practice. I gave up trying to somewhat automate or remotely do the switching within the house.

Carl

Yep Carl, the situation for hams is more involved for certain. You have to worry about sending as well as receiving among other things.

I was pretty sure that this idea was going to have its limitations. I have not actually measured intermod but I haven't seen any yet. Down on the NDB band, I have to be very careful because there are many BC stations near me and the products are all over the band. I use LPFs and Band Reject filters to keep some of that under control. Not much you can do with the active antennas so mine doesn't get used much, and when it is used it is after dark when the big stations reduce power.

So far everything seems great for SWL except the increased broadband noise. In theory, preamps can fix that but it might make the intermod problem even worse.

I did some measurements. At a 1KHz bandwidth, the switch noise floor is at about .8 microvolts. At a 10KHz bandwidth its more like 2.5 microvolts. My spectrum analyzer won't do bandwidths in between 1 and 10 KHz.

As for intermodulation, I'm not really setup to do a real measurement so I just ran the longwire straight in and looked for products in the longwave band. I don't really see anything. The strongest carriers in the BC band are at about -10dbm (50 ohm system). If I played with it I could probably bring that up some but that is a pretty substantial signal as it is. I am guessing that the intermod is low because the video channel gain is 0db.

So, for my purposes, I think that this is going to work out OK.

Tony

Check out http://www.w8zr.net/multicontroller/index.htm. It will handle a large number of receivers and one high-Z and one low-Z antenna. Switching a larger number of antennas can easily be handled with a low-power coax switch or the arrangement of your choice.

Jim isn't selling the parts kits any longer, but if you keep an eye out you might find an assembled unit. Pricey but it's what you're looking for.

Neat looking box Avery, but it only has one antenna input. I want a number of antenna inputs that can be switched to multiple receivers in any combination.

Tony