split antenna feed

[davecason]

Hi,

I have a new antenna up on the roof, it's a VHF/UHF six foot fiberglass mast, and I have the feed line running down in to a room in the house. Using a product like RF Systems SP-1 backwards like they suggest, can I split that feed and run a second feed line off that "splitter" in to a different room in the house so I can move the radio around?

OPT A. 1 antenna feeding from 2 locations.

Or could I have the radio on one line and a scanner on the send line? I don't think so because the radio would cook the scanner if I tried to transmit on the radio while the scanner was plugged in right?

OPT B. 1 Antenna feeding an HT radio and a scanner.

Cheers'
Dave

 

[ko6jw_2]

Splitter issues

I looked up the SP-1 on Universal Radio's web site. It seems to be intended for use below 35Mhz. It is true that splitters can be used in reverse (usually), but the cut-off frequency indicates that it would not be useful for VHF and UHF. Assuming you have a suitable splitter I would suggest an amp in the antenna feed before any splitters. The more you insert the greater the loss. This may be a moot point if you're listening to strong local repeaters, but for weaker stations it could be a problem. Take a look at splitters, combiners and duplexers from Comet or Diamond. They are not cheap, but are designed for VHF and UHF.

 

[WA0CBW]

You need a diplexer, not a splitter. A diplexer is a two port device that isolates the two ports from each other. This prevents the transmitter on one port from feeding back into the 2nd port. Diplexers come in various bands and signal splits. Comet or Diamond have various models that come in various band and frequency splits that would serve your needs.

 

[davecason]

Ah ! Thanks

OK, so I need a diplexer and not a splitter ......

Can anyone suggest a model?

Keep in mind this is just for a HT plugged in to a VHF/UHF 6 foot antenna up on the roof. It's works fine upstairs where I ran the coax too but I'd like to be able to take it downstairs in to a different room so I can use it in two locations with the same antenna. The diplexer itself would have to be outside up near the top of the roof where I would split the feed and run each piece of coax to the respective rooms.

If I get rid of option two and just use one radio on the ends of the coax at different times would a splitter work then or again is a diplexers the way to go? I still think it would because of the fact that there is an open length of coax just sitting there unterminated.

Am I on the right track now ..

Cheers
Dave

 

[mmckenna]

Not quite.

A diplexer splits one antenna to two different band radios, or one radio to two different band antennas. You -could- use a diplexer, but you'd need to run one band at one location and a different band at the other location. Trying to backwash 2 meters into the 70 cm side of the diplexer is not going to work well. Same goes for trying to run 70cm into the 2 meter side.

A traditional passive splitter isn't going to work either. Neither will a "T" connector. Both of these will be splitting power between the antenna and an unterminated point.

What you need is a manual antenna switch. The antenna comes into the common port and there are two outputs to feed your two radio locations. The issue with this is that if you forget to set the switch, it isn't going to work. There are some "safe" switches that will allow installing a terminator that the unused radio port gets switched to. Might be a good option if you think you might forget to switch it.
For VHF, here is an automatic switch that senses when one of the ports is being used to transmit. It's designed for marine VHF, not sure how well, or even if, it would perform on UHF: http://www.amazon.com/Shakespeare-Auto-Switch-Radio-Ant/dp/B000GFWWMS

Other option is two antennas, but understandably not what you are looking for.

 

[davecason]

Hi Mmckenna,

Thanks for the info .....

But yea, see that's what I was afraid of ..... I know I can do it with as switch but the problem with that is, I want to make the point where the split occurs to be outside up on the roof. One side will run across toward the back of the house and in to the bedroom window where it is now and the second feed running the exact opposite direction towards the front on the house and in to a different window.

Thus the idea split is not exactly where I can get to it. (grin) I've seen and heard of those auto sencing switches as well but again not really what I had in mind. Maybe I'm the only one who wants to use the radio in two different locations in a building .....

I think I'm pooched! I'm gonna have one ham shack in one location in my house and learn to like it by god ! (grin)

Here's another one as well right now the feed line is Jetstream JT-1908 and I just bought my first HF rig, an Icom IC-745 so I have to get a bunch of Belden 8214 to feed the G5RV I'm going to start out with ...... do you think I might as well swap put the Jetstream for some of that same 8214 - it'd be another 60 ft purchase? Swap it out or leave it?

The jetstream stuff is a like a Mini 8/U equivalent to RG-8X and of course the Belden is RG-8. someone said to use Belden 9913 over the 8214 ...... What do you guys think?

Here's the next question too as far as the G5RV goes - do they all have that 30 ft ladder tape on the antennas to match or can it be made shorter?

Cheers'
Dave

 

[mmckenna]

Try this site:
Coax Calculator
You can play with cable type, length, SWR, etc. and figure out your losses. Not sure who makes the Jetstream stuff, but if it is a Mini 8, you would be hard pressed not to do better.