Sending power up the mast

[jassing]

I am building a remote switch to select vertical or horizontal antennas. It works off 5v and so far, it works well. Now i need to put it up at the mast and control it from the house.
The distance is too far for DC current I think, so I was thinking of using a 110vac->15vac transformer and then using a rectifier/capacitors to get about 12vdc. The question is, do I need to have the a/c power separated from coax, or can it be near the coax if the a/c line is shielded?
I am running RG8 from house to antenna, and would rather not have to buy 2x as much to have an indoor/non-remote switch. (I have all the parts & wire for the remote switch, so it's not costing me much, if anything, other than time)

 

[cmdrwill]

I am building a remote switch to select vertical or horizontal antennas. It works off 5v and so far, it works well.

I would think it does not draw that much current being 5 volts. I would use something like zip cord and stay at 5 volts. If you run it near the feed line/coax and this is a transmit antenna, use ferrite cores on each end to keep RF out.

 

[prcguy]

Is this for HF or VHF/UHF? Its really common these days to run voltage up the coax using a "bias tee" or voltage inserter. For VHF/UHF you can use one from a cheap TV antenna preamp or you can make them for just a few $$. I'm with cmdrwill that your switch may not draw enough current to worry about voltage drop.
prcguy

 

[jassing]

No, HF antennas.
I hadn't thought about running it up the coax. But that's only D/C right?
So 100' and 5vdc isn't too far? Seems when I built our last house, I had problems running 12vdc to all the outlets, so just assumed 5vdc would be even harder to get that far.
Thanks for the tip on the ferrite core.

 

[KC4RAF]

Use ohm's law to calculate

what the voltage drop will be for that distance. You will have some drop in that line, depending on the size of the low voltage wire. R=E/I; I=E/R; E=IxR
100' is quite a distance for 5dc.
Keep us posted how it works out for you.

Here's a link to help figure the Vd:

Voltage Drop Calculator

I "assume" (bad to use assume my friend, but I did any way), you'll be using #14 copper zip cord. Probably 1 volt drop.

 

[jassing]

thanks for that link. Well; now I'm stumped as to why I had it in my head a/c travels better than d/c - that shows them to be the same. Funny how memory fails us on things we thought we knew to be so true.

Thanks!

(ps: I dig your tag line -- My pro2026 still works great, out here in the rural area, no trunked systems!)

 

[KC4RAF]

Thanks for the comment on my tag. lol

There IS a difference in ac to dc laws. But in the case as yours', it's not big a deal.
I think you would be ok with running a wire up the mast for the relay to operate; the small amount of voltage drop isn't going to be that large. But by all means, keep us posted on your project, and ask questions if the need arises. We like to hear how some body is doing with things like this. Plus we all learn from what YOU are doing by giving us answers.

BTW, ac does travel greater distance with less loss due to effects of alternating current flow. DC will loose a lot in a long run. The other thing is that ac is easy to either reduce or increase the voltage by use of transformers. You can not do that easily with dc. So you are right in what you thought, but in your case it's too short a run to worry about. HTH some. lol

 

[prcguy]

A bias tee or voltage inserter is really easy to make for HF. I can't find a good diagram with component values so I'll try to describe one.

Start with a small box, preferably metal where you can mount two RF connectors opposing each other maybe an inch or two apart with their back sides facing each other. I use cast aluminum boxes from Hammond or Bud that are about 4" long X 1.5" high and maybe 1.5" wide. Bridge the center pins of the connector with a .01uf disc cap and the voltage will depend on if your receiving or transmitting and the power level. You can't go wrong with a 1kV rated disc cap or several smaller value caps in parallel to add up to about .01uf. For receiving only, a 100v rating is fine.

Then solder a small inductor between about 75uh and 100uh to one of the connector center pins with the shortest lead length you can make and at a right angle away from the cap that bridges the two connectors. The inductor needs a current rating of a least what your relay is expected to draw. I use JW Miller model 5250 rated at 100uh at 2A because I got a case of them cheap on Ebay. On the other end of the inductor solder a .01uf disc bypass cap from the unused inductor lead to the metal box (ground) using the shortest lead lengths you can.

Finally, install whatever kind of power connector or screw terminal you want on the box to feed in your voltage to the inductor/bypass cap junction. Better yet, try to find a .01uf feed through capacitor, the kind that threads into a metal box and has an input and output connection and that can be both your inductor bypass cap and power connector. Make a second box just like the first.

The connector that has the inductor attached will be where your feedline connects at each end and that's where the power will be injected or pulled out of the coax. The connector with no inductor will be what connects to your radio(s) on one end and the rest of your antenna switching stuff at the far end. The cap(s) between the connectors block your inserted voltage from getting to your equipment or antenna. The inductor with bypass cap provides a low impedance path for the voltage to get into the coax while providing a high impedance to the RF so it doesn't flow back into your power supply and so the power supply doesn't load down the RF path.

I've made lots of bias tee's for HF through UHF and use them to feed power to remote HF antenna tuners, mast mounted preamps, etc. A bias tee as described will pass AC or DC voltages up to near what the disc caps are rated for. If you can't find the parts locally they are all available on Ebay cheap.
prcguy

No, HF antennas.
I hadn't thought about running it up the coax. But that's only D/C right?
So 100' and 5vdc isn't too far? Seems when I built our last house, I had problems running 12vdc to all the outlets, so just assumed 5vdc would be even harder to get that far.
Thanks for the tip on the ferrite core.

 

[prcguy]

Forgot to add, since you need 5v you could probably use a 6v wall wart and on the bias tee at the radio end use a coaxial power socket on the bias tee box that matches whats on the wall wart.
prcguy

 

[AK9R]

If you don't feel that building a bias T is in your skill set, MFJ sells them:

MFJ Enterprises Inc.

 

[kj3n]

I'm curious as to why 5V? Most remote switch applications I've seen use 12V, which would help offset the voltage drop on a 100 foot run.

I've used a couple of different mast-mounted RX preamps on VHF and they've all used 12V.