Vacuum relays in antennas for band switching - question

[steve_qix]

Hi !

I have a situation up on Rattlesnake Island where I have up a very nice, full size 75 meter dipole up at around 70 feet.  Coax fed - the antenna works very nicely, and fits completely on my property.

I need to get on 160 meters, so as a test I attached a 65 foot long vertical section to each end of the 75 meter dipole.  The antenna seems to work very nicely on 160.  I'm net control for the Grey Hair Net tonight, so we'll see.  I just put the antenna up yesterday.  Of course, now I can't get on 75 meters anymore.

The goal is to try to keep the dipole antenna for 75.  It works very well, and I don't seem to interfere with anything around here.  There is insufficient space to put up a separate 160 meter dipole.  Running open wire line is impractical for a number of reasons - and I really like the dipole configuration and radiation pattern.  Also, don't want those vertical sections involved at all when on 75 meters.  Unfortunately, the "2 dipoles on one feedline" thing won't work here - physical restrictions and power lines prevent that configuration, or I would do it.

So, my idea is to simply place high voltage vacuum relays at each end of the 75 meter dipole, and switch in the extra 65 foot vertical sections at each end when 160 meter operation is desired.

There are some Gigavac 15kV DC (about 12kV on 75 meters) vacuum relays on Ebay that might just do the job.

Has anyone here had any experience doing this sort of thing?  Is 12kV enough?  I'm running 400 watts carrier output with modulation capabilities of 200% positive from the Island.

Anyone heard of Gigavac and/or have any HV vacuum relays they would like to part with reasonably?

Thanks and Regards,

Steve

[Bill, KD0HG]

Steve:
How about using traps?
You can make lightweight capacitors out of sections of RG-213, or use that pricey RG-8 sized teflon coax for more bulletproof caps. You can find it surplus or at hamfests.

Bill

[WA1GFZ]

Too much voltage at the relay contacts on 75M. I will be very interested in your performance on 160 having the same issue at my beach place.
But i have no problem using open wire line and don't see how it will effect radiation pattern.
RG141 0r RG300 might also work as traps. RG393 runs about $5 a foot. How about 1/2 inch andrews hard line??

[steve_qix]

Hi Frank,

How much voltage do you think there will be?  I figured the impedance at the end of the antenna to be less than 50,000 ohms - let's say it IS 50,000 ohms.  A 400 watt transmitter modulated to 200% positive should produce, at 50,000 ohms, around 13kV - that's worse case all the way, of course.  The transmitter actually won't be modulated 200% positive, but I want to have a good safety factor - even if the antenna's impedance at the end IS 50,000 ohms.

Am I off on my impedance number?  I figured this once, but that was a long time ago and I can't at this point remember how I did it 

[WA1GFZ]

I think the voltage will be higher, then consider induced lightning. then you would then need to control the relays
How about an inverted L for 160. pretty easy to get to coax feed You should be pretty efficient being close to the water.
Make it a bit long and resonate it with a series cap. A Top loaded T would do the same thing.

[KM1H]

Ive seen 80/75M yagis use vacuum relays to switch segments but they are only part way out on the linear loading where the voltage is lower.

For a dipole I only see something like 3 half waves in phase on 75 which would be the full antenna and breaking it down shorter for 160 which would make the relays at a midway point of the 2 outer segments.

Coax can feed both with reasonable VSWR....such as using a 40M dipole on 15.

Carl

[WD5JKO]

I found a good discussion on this very issue, and several alternatives are mentioned:

http://lists.contesting.com/_topband/1996-11/msg00018.html

Jim
WD5JKO

[W4NEQ]

Couple thoughts here:

Unadilla traps are rated for 2kw pep, and if used, you could stretch the bandwidth just a bit with a line tuner.

According to the Unadilla web site:

"condensers are epoxy-sealed"  suggesting store-bought caps?  It's rare to see small store-bought caps in excess of 20kv, so that may be the realistic maximum voltage.  Then again, that application has a coil shunted, reducing the voltage somewhat.

If you made the ends of the 80 meter dipole portions cage-dipole fashion, you would lower the Q a bit and reduce the end voltage, improving the vac relay safety margin.

Chris

[WD5JKO]

Unadilla traps are rated for 2kw pep, and if used, you could stretch the bandwidth just a bit with a line tuner.

"condensers are epoxy-sealed"  suggesting store-bought caps?  It's rare to see small store-bought caps in excess of 20kv, so that may be the realistic maximum voltage.  Then again, that application has a coil shunted, reducing the voltage somewhat.

   I have a set of Unadilla KW-40's, and back in the 1980's I ran my 1KW DC input AM rig into that trap dipole. This rig put out 850 watts carrier, and modulated fully, so the traps took > 3 KW PEP for many years. The capacitors seem to be just aluminum concentric tubing with epoxy sealing the gap. I still have those traps, and they are outside. So after nearly 30 years, I only see a few cracks in the blue coil support plastic. Those things are tough.

Jim
WD5JKO

[KM1H]

Ive been running 160/80 inverted vees with a common feedline since the early 70's when working temporarily in IL. It was written up later in the YCCC monthly after I explained it to Randy, K5ZD/1.

At this QTH I rebuilt them 2 years ago (20 years of lightning strikes pretty much destroyed the 7 strand copperweld) and added another for 75 so I get a nice letter W VSWR pattern. It required some trimming to get the 50-75 Ohm point where I wanted in order to optimize the bandwidth for each segment which is about 150KHz for both at the 2:1 points.

Traps will likely narrow that bandwidth.

[W4NEQ]

Now that I think about it, two dipoles on a common center insulator might be the better solution.  The greater the angle between wires, the less they interact and the better the bandwidth.

Chris

[WA1GFZ]

I have seen multiple dipoles off a common feed work with only a few feet of spacing at the ends. Tricky to trim but quite usable. I ran 3 half waves in phase on 40 and an 80 meter dipole coax fed once.