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Author Topic: Double Pole - Double Throw Switch for Window Line  (Read 3679 times)

Posts: 38

« on: August 18, 2008, 07:07:50 PM »

I am looking for ideas for a switch to select between two different antennas fed with twin-lead, window line, or ladder line.

I have a NyeViking Antenna Matchbox.

I suppose I could use a double pole double throw knife switch.  But I don't like the idea of exposed, energized metal.

Are there insulated switches I could use?


73, de K5RFF

Posts: 2086

« Reply #1 on: August 19, 2008, 07:04:19 AM »

Sure...any builders supply will have knife switches in a nice NEMA box.

73 de Lindy

Posts: 625

« Reply #2 on: August 19, 2008, 09:16:37 AM »

I assume you want to make the switch have approx same impedance as the window line.

Here's what I have done.  I use double banana plugs that are .75 inch center to center.  Then I add banana jacks to the tuner outputs and just plug in the desired antenna.  Banana jacks and plugs make very good balanced line connectors.

You could also use two single pole switches (toggles etc) and space them about the same as the window line and throw both to change antennas.

You could scavenge an old wafer switch with about the right spacing and use it to switch both conductors.  You could probably have more than two positions as well.

You could get some relays and space them about the same as the ladder line and switch the coils on and off.

If you aren't really so concerned about maintaining impedance, you could just use any sort of double pole switch, where the contact spacing could support the RF voltage.  If you are using a "tuned" line the SWR is maybe high anyway and the impedance bump would not be of any concern unless it results in a hard to match value.

You can put what ever lashup you come up with in a plastic box, say like one of those utility boxes that are used for electrical boxes or utility connections.  Use the banana plugs and jacks on the lines and you have a window line switch.

I really don't see any need for knife switches except for historical reasons and they have about the same spacing as window line.

Posts: 21764

« Reply #3 on: August 19, 2008, 09:53:17 AM »

Almost anything that can handle the voltage (when open) will work.

Maintaining impedance through the switch, for HF band operation, doesn't matter.  If the impedance "bump" at the switch changed the Zo from 450 Ohms to 50 Ohms or to 1500 Ohms for the length of the switch (about an inch), even at 28 MHz this is .0025 wavelengths, or less than one degree phase difference.  On lower frequencies, it's less.  It just really doesn't matter.

Same reason that the impedance of a PL-259 doesn't matter until you get to about 300 MHz.  It is an impedance discontinuity, and can be a fairly large one, but it's so short it has no impact on anything.

I've used old fashioned hardware store knife switches for this application for 40 years and they work fine: They're just getting hard to find nowadays!  If you want to "remote" the switch just using a DC relay, the important parameter (assuming you never try to "hot switch" the relay with power applied!) is standoff voltage; that is, the withstanding voltage of the contacts when they're open, and also the withstanding voltage of all contacts to the relay coil or other conductors nearby, and also whether the relay is rated for continuous activation (some are not).  Current carrying isn't so important, since 10A contacts are pretty small and easy to find.


Posts: 10

« Reply #4 on: August 25, 2008, 12:43:29 PM »

I picked up a couple of DPDT knife switches on Ebay that are WW2 military surplus that were apparently used for antenna/radio/ground management as you describe.If you can get over the exposed metal [I mounted mine outside in a MTM Sportsman's Dry Box on a piece of plywood with standoffs under the switch to allow passage of wiring.You can lock it] they would be perfect. Just use the Ebay search engine to look for them. You should descibe them in several different ways as I have found my description did not always match how the item was catalogue. Look for them to cost you arould $50 +/-. Good luck.

Posts: 214

« Reply #5 on: September 03, 2008, 08:19:56 PM »

I found a switch at Lowes made by Hubbel, 20 amp DPDT used for house hold wiring. I put it in a plastic wall box. It is mounted on the feed thru panel at the window, I saw no tuning change after installed. I am feeding it with six feet of 450 ohm ladder line from the tuner.
BTW>> some switches have silicone grease < from the factory >  "hidden" inside  them ,,,look out for that.

Posts: 642

« Reply #6 on: September 09, 2008, 12:55:12 PM »

I installed banana jacks and plugs on a piece of plexiglass then mounted the plexiglass on stand offs to the out shack wall. I also added the option of grounding everything when not in use.

Posts: 378

« Reply #7 on: February 05, 2009, 11:46:28 AM »

I've recently see one company offering 'surge protectors' for ladder or open wire feed lines.  One look at them shows you how simple this is.  They're spark plugs!  

Yup, 2 simple ole spark plug you'd use in your lawn mower mounted on a piece of metal (aluminum or copper) with a nut holding it in place.  Space them the right width for your line, and you're all set.  No knife switch, no forgetting to throw it.  Automagic.

Look for the spark plug with the screw on adapter at the wire end.  Then remove and replace with a couple of washers and a nut, wrap the wire around the post and tighten the nut.  Almost too simple.
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