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1-6 of 6 messages
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Switching box for phased array
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by KJ4KET on June 16, 2009
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I have had a great discussion thread going in the Tower Talk forum on the possibility of building a three phased vertical triangle array in my back yard, for 20m. However, unfortunately this all has to be done on a shoestring (kids in college, etc). I was looking at commercially built phasing and switching boxes, but they cost a small fortune, so I would like to build something myself.
There are two designs we have discussed for the array: One feeds all three verticals simultaneously, two at half power relative to the other one (with three possible switched beam directions); and the other only feeds two verticals at a time, with six possible switched beam directions. In each case, phasing and impedance matching is largely done through the use of phasing and matching cables.
Thus the purpose of the box I need to design would be merely to make sure the cables are switched to the right verticals.
What I am worried about is the selection of switching components where RF is being switched. I could just rig up something like an old style phone switchboard and do everything manually, but that would be a nightmare, to run out into the garden every time I want to change the beam direction. Ideally, I would build a remotely controlled box that uses relays to switch between the antennas. I could have a control box in the shack with a six-way (or three way) rotary switch applying a voltage to cat 5 cable, and the cat 5 would energize the correct relay in the antenna-mounted switching box.
If this all seems to make sense, the issue becomes: What sort of relays can be used for RF switching functions like this? I'm assuming that I may want to use this with up to 600w PEP output. Even if I scale back to a two vertical system I will need some sort of switching arrangement.
Any design hints/circuit diagrams/part numbers/practical tips would be most welcome.
David M
KJ4KET
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RE: Switching box for phased array
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by KG4RUL on June 16, 2009
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High Voltage, Vacuum relays can be obtained from MFJ - see their parts section, page 73, in their catalog. If you don't have a catalog, you can download the section in PDF format at: http://www.mfjenterprises.com/catalog/Pages_51_through_75.pdf - Similar relays can be found at other sources.
Use a 6 position switch to activate pairs of relays for each direction selected.
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RE: Switching box for phased array
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by N3OX on June 17, 2009
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If you've got a good match, even 600w into near 50 ohms is likely to be under a couple hundred volts. The RF breakdown of air is lower at RF than at DC, but vacuum variable capacitors seem to be derated by a factor of two for that reason.
I think relays rated for 480V AC @ 60Hz would be totally suitable for your application with a rather healthy safety margin, and you don't really need anything special at HF. And honestly, even 250VAC relays are probably OK.
I'd probably go for something rated 5A or higher.
It's good to try to keep leads as short as possible, but for HF it's not too critical.
A lot of commercial switches use pretty stock PCB mount relays, something like this:
http://relays.tycoelectronics.com/schrack/pdf/RTH10516A.pdf
Here's some more info:
http://www.ad5x.com/images/Articles/AntSwitch.pdf
http://www.iol.ie/~bravo/remote.htm
and here's some info with measurements of the finished thing and discussion of RF suitability of simple relays:
http://www.qsl.net/k5lxp/projects/CoaxSwitch/CoaxSwitch.html
Don't buy vacuum relays for 50-ohm impedance switching, it's a waste of money. They're excellent if you need to switch thousands of RF volts, but you don't need to do that.
73
Dan
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RE: Switching box for phased array
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by KJ4KET on June 17, 2009
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Thanks for the great information! Dan, it seems like I can save a ton of money by using the type of relays you have described.
My project will involve switching phasing among antenna elements rather than switching the transceiver among antennas, but I think I have found an appropriate circuit for this, and the info you have provided has helped greatly in assisting me in selecting the right components.
Thanks again to all for the valuable comments.
David M
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RE: Switching box for phased array
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by N4JTE on June 22, 2009
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David; have not had any problems to date using a $8 radio shack dtdp relay for the phasing systems I run here at 800 watts.
Bob
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RE: Switching box for phased array
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by 2E1FVS on June 30, 2009
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the triangle array sounds interesting, i'll have to go and have a look in the tower talk forum.
but my OM, John, G4ATA, reacently built a remote switch box for his pair of phased verticals for 40m. some info on his web site. http://www.g4ata.co.uk
I hope it's of some use and you get a few ideas!
73 Martin, 2E1FVS
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