Antenna separation uhf/vhf

[wqzw301]

Hello All,

 

I could use some expertise.

 

I'm putting up a mast on my roof -  8ft  fiberglass.

 

I am mounting a j-pole on top, tuned to my repeater frequency...  

 

I also have a 1/4 wave marine vhf, I was going to mount below the uhf j-pole.

 

But I started thinking....the marine antenna would be use,  only sporadically,  to contact my brothers boat. The antennas would not be used simultaneously...vhf mostly for bad weather check ins....

 

Can I mount them side to side on top of the pole?  The pole has a  1 1/2 dia. and I can mount both without contact.

 

I know vhf antennas should be on top of the stack, but the vhf is just an addition and I have great line of sight to the ocean...but can they be mounted side to side? and if so -  One radio would have to be off while transmitting on the other, because I would be putting rf down the other radios coax  -correct????

 

thanks for taking a look guys

301

[PastorGary]

There are options.  This is just one that I have seen successfully used in a similar application to yours. IF you can mount the VHF antenna on an outrigger 3 feet from the mast and below the UHF, with the majority of the VHF antenna facing the water, you would most likely have no or very limited interaction.  With BOTH mounted at the top, essentially side by side operating on two different bands, there would probably be some interaction including de-sense on the radio not being used in transmit mode as well as some involvement with a higher SWR for both.  Vertical separation of dis-similar antennas is usually the way to reduce issues.

[wqzw301]

PastorGary

 

thank you very helpful.

 

I think I've heard somewhere that every foot of vertical separation is equal to 10ft of horizontal separation ???

But I'm not sure if that applies to every frequency....

 

thanks

301

[PastorGary]

John E  may have that formula somewhere - but that sounds close.

[berkinet]

I would also suggest using a directional antenna for the VHF radio. Since Marine VHF is only legal to use when communicating between or with a boat, unless you live on an island, it is likely the water is within a specific arc from your home. Depending on how wide that arc is, you should be able to find a suitably matched VHF directional antenna.  Any propagation loss you get from mounting the antenna below your UHF J-Pole would be offset by the gain.  You should also look for an antenna that has low radiation upwards or downwards, since most boats tend to be at water level . And, make sure you mount it vertically.

http://www.antennaskit.com/files/Yagi-4-elementi_30_150-300.JPG

 

 

You can also use an outrigger as PastorGary suggested. That would work must better with a directional antenna than an omni, since the antenna mast itself  would not be in the path of your beam - and would, probably, act as an additional reflector..

[wqzw301]

I am surrounded by water.... Bay and ocean. Would the directional antenna be best? I will not be able to turn it....

I have a small skiff and my brother has a commercial boat. That's the only reason I started to consider mounting the vhf..It was an afterthought.

Also I know the fcc just allowed marine vhf handheld portables to be used from shore to ship. But I'm still going to apply for a marine license for base station, with my frn on the fcc licensing sight.  I just have to see what, or if any commercial info I have to provide...

 

I would like to use maybe even experiment with building a directional, but on the roof, with no access??? 

 

thanks

301

[berkinet]

I am surrounded by water.... Bay and ocean. Would the directional antenna be best? ...

When I mentioned water I was really referring to where the boats that you are going to transmit to would be located.  If you will be transmitting over more than 180º, then you should probably stick with an omni.

 

But, I re-read your original post...

 

....I'm putting up a mast on my roof -  8ft  fiberglass.

I am mounting a j-pole on top, tuned to my repeater frequency... 

Are you going to use GMRS only through the repeater, or will you also want broad omni-directional coverage?  If you are only using the repeater, than a nice compact UHF yagi might be a better choice.

 

If you do need the j-pole,then,  since the mast is fiberglas, you should not have any issues mounting the j-pole along side the mast, with the VHF omni-antenna on the top of the mast.

 

Take a look at this web page for more information on mounting j-poles.

[JohnE]

like PG said

since they will most likely will not be used at the same time you could get away w/side arming the shorter of the 2 antennas. as long as the VHF is not obstructed to the water you should be good.

I have to go look for the separation chart , it's been a while.

[berkinet]

since they will most likely will not be used at the same time ..

Until the day they are, like when he needs to relay messages between a boat and a land-mobile user. Of course, it would be quite unlikely to transmit on both at the same time. Still, it never hurts to plan for robustness.