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Old 05-06-2017, 21:42   #1
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Multiple antennas at mast head

So my mast is down and I am mounting a new marine VHF antenna on the mast head. I am seriously considering the purchase of a 2m dual band vhf/uhf radio and would like the groups advice on the mounting of antennas for both at the mast head. They would end up 12 to 18 inches apart with one mounted forward and one mounted aft.
TX on one radio at a time other would be turned off.
Am I completely nuts? Would mounting the dual band on the spreaders be better?
Boat is a Lagoon 380 if it makes a difference

Thanks
Derick
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Old 06-06-2017, 04:49   #2
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Re: Multiple antennas at mast head

Quote:
Originally Posted by DRS View Post
So my mast is down and I am mounting a new marine VHF antenna on the mast head. I am seriously considering the purchase of a 2m dual band vhf/uhf radio and would like the groups advice on the mounting of antennas for both at the mast head. They would end up 12 to 18 inches apart with one mounted forward and one mounted aft.
You'll end up with directivity fore and aft.

Just like the reflectors and directors on an old TV antenna provide a beam in one direction so will your two antennas separated by close to a quarter wavelength.

You'll get more consistent omnidirectional performance for your marine VHF by giving it pride of place at the masthead. Put your dual-band on a spreader, radar mast, arch, or pushpit.
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Old 06-06-2017, 04:58   #3
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Re: Multiple antennas at mast head

I build TV antennas out of alloy tube inside fibreglass tube mounted at the masthead. The design is vertical quarter wave dipoles which generally give good reception. What would be the effect of running the coaxial feed line of a stacked set through the middle of the lower antenna.
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Old 06-06-2017, 05:13   #4
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Re: Multiple antennas at mast head

I have an antenna for dual band VHF/HF ham radio, and I mounted it at the first spreader to avoid conflicts with the marine VHF antenna.

I use this antenna with my AIS set; it can be switched with a Bird coax switch. It works well.

Don't put it up on the masthead truck with your main VHF antenna.
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Old 06-06-2017, 05:57   #5
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Re: Multiple antennas at mast head

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Originally Posted by RaymondR View Post
I build TV antennas out of alloy tube inside fibreglass tube mounted at the masthead. The design is vertical quarter wave dipoles which generally give good reception. What would be the effect of running the coaxial feed line of a stacked set through the middle of the lower antenna.
Good quality shields in the coax are important.

I'd put the VHF over the TV (mission over entertainment).

Given the huge frequency range of TV what does "quarter wave" really mean?
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Old 06-06-2017, 12:01   #6
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Re: Multiple antennas at mast head

I have had 3 antennae at the mast head in a t pattern with each about 30" apart. They are marine vhf, 4 band vhf ham and an AIS b. While there must be some interaction in practice I can't detect it. I run 100 watts on 29, 2, 6 and 70 without any damage or impact to other radios.

Mini 8 and westflex 103 are the coaxes used.

We have a Tayana 47 with tall rig and the antenna are about 72ft above sea level.

Do they sing ? You bet, nice and clear of rigging gives a much better pattern and height matters.

I originally thought the T arms may look a bit silly but most people barely notice them.
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Old 07-06-2017, 06:32   #7
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Re: Multiple antennas at mast head

Quote:
Originally Posted by DRS View Post
So my mast is down and I am mounting a new marine VHF antenna on the mast head. I am seriously considering the purchase of a 2m dual band vhf/uhf radio and would like the groups advice on the mounting of antennas for both at the mast head. They would end up 12 to 18 inches apart with one mounted forward and one mounted aft.
TX on one radio at a time other would be turned off.
Am I completely nuts? Would mounting the dual band on the spreaders be better?
Boat is a Lagoon 380 if it makes a difference

Thanks
Derick
No, you are not nuts.

Without knowing the type of antennas (collinear/1/4 wave/ etc) being considered it is impossible to be certain one way or the other and of course more separation is always better, especially vertical separation.

BUT....

You have good frequency separation (~160 MHz to ~450MHz).
Reasonable horizontal distance separation (18")
And for both bands, height trumps power so any loss from the close proximately is more than made by the having both antennas at the maximum height.

So if it was me, I would have both on them at the top of the mast with perhaps a small amount of vertical separation as well. As the UHF antenna will be much shorter, it might not be hard to fabricate a mounting so that the VHF antenna is just above the height of the UHF antenna and still keep the 12 to 18 inches of horizontal separation.

And to repeat, height trumps power (including ERP and EIRP).
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Old 07-06-2017, 07:12   #8
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Re: Multiple antennas at mast head

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Originally Posted by Wotname View Post
You have good frequency separation (~160 MHz to ~450MHz).
Reasonable horizontal distance separation (18")
No. 2m ham is 144 - 148 MHz. Marine VHF is 150 - 160 MHz.

18" is a quarter wavelength (ish). Not good. There will be interaction just by virtue of the presence of the metal in the other antenna. The OP WILL get directivity effects.

It's a poor choice. Put the dual band ham on an upper spreader.
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Old 07-06-2017, 08:01   #9
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Re: Multiple antennas at mast head

Derick,
1) I've written about this in detail in the past....and while a 3' to 6' horizontal spacing is considered the minimum at vhf high-band (with a few MHz of separation), to allow the radios to operate simultaneously, you can use closer spacing, and accept receiver-desense, etc...
(Read below, for a LOT more info...


2) Some brief pieces of info/fact upfront:
a) makes no difference if the radios are on or off...
b) the actual installation of the cable, and especially the connectors, will have the largest effect on performance (and longevity!)
c) use of good quality cable and connectors is also of more importance...(Belden, Times, Davis, Commscope, etc...and Amphenol, Kings, etc.)
d) do not be lead down the path of using what some guy at the store (or worse, some guy down the dock!) tells you is best....
(RG-213 is great! RG-8x is "okay", but not preferred....if you cannot fit RG-213 inside mast conduit, and you feel you must have lower loss, then you can use LMR-240UF, instead of RG-8x....but do not use any "LMR"-type cables unless they are the "UF" designated...and use of LMR-400UF is quite overkill, waste of $$$ and a pain in the but!!...
So, use RG-213 is best!!)
e) properly secure the cabling in conduit or with a LOT of cable tie-wraps to securing screws, etc...
f) properly weatherproof all connections!!
g) read the other threads where we discuss cable losses, etc., as well as radiowave propagation, etc...


3) In your application, you should have no major issues...desense of the receiver connected to the adjacent antenna, when transmitting with the other radio is a given, and if transmitting / receiving on vhf, the adjacent receiver will most probably be blanked/blocked while transmitting on the other radio...but, that's probably all the issues you'll have...
(If you plan on using the ham rig a lot, definitely put it's antenna on a spreader, that way you'll not be interfering with your primary VHF's reception!!
And, as I wrote, this is probably the best approach...but, your approach is do-able..)

While some might think having another antenna close (12" - 18" at vhf is close) would produce significant directivity, since you'll be using a multi-band antenna which will have its resonances well away from the VHF Marine Band, it is unlikely to be significant...
Although, the pattern from each antenna will be effected by this other antenna in close proximity, it should actually be minor (a db or so advantage in one direction and db or so disadvantage in other direction), and the changes in antenna patterns by the action of the boat/mast (waves, heeling, etc.) as well as the wind whipping the whip around, will be much more significant than the effect of the other antenna....
[note that the actual resonance of the adjacent antenna is unknowable to us, as it is controlled not only by the specific antenna and its feeding/matching (especially the multi-band design loading/matching), but also by the feedline attached to the antenna (which actually "tunes" this antenna when being radiated parasitically)...so, it's unlikely to produce significant directivity....{ btw, decades ago, I installed a switched/multi-directional vertical array that used the unused feedlines (from the other antennas) to tune them off-resonance and allow them to act as parasitic reflectors, actually giving about 6 - 7 db of gain....} ]

Quote:
Originally Posted by DRS View Post
So my mast is down and I am mounting a new marine VHF antenna on the mast head. I am seriously considering the purchase of a 2m dual band vhf/uhf radio and would like the groups advice on the mounting of antennas for both at the mast head. They would end up 12 to 18 inches apart with one mounted forward and one mounted aft.
TX on one radio at a time other would be turned off.
Am I completely nuts? Would mounting the dual band on the spreaders be better?
Boat is a Lagoon 380 if it makes a difference

Thanks
Derick
4) Most of the above notwithstanding, placing the dual-band antenna on a spreader would probably be better...and this would be my advice!!
BUT...

But, I have your exact proposed installation on my masthead, and have had it all there for many years, without issue...(about 12.5 years, numerous gales, 3 hurricanes, etc...multiple cruisers, including two Atlantic crossings, etc...)
So, this approach is do-able....not recommended by most, as it does need to be understood...but it is do-able...
At my masthead I have one Shakespeare 3' VHF SS Whip antenna and one Tri-Band vhf/uhf ham antenna, mounted on opposite sides of the mast...
(the Shakespeare is on the starboard side, just aft of center....tri-band ham antenna is port-side, just forward of center...have a look at pics)


I have noticed no significant directivity at all!!
Please let me repeat this...I have noticed no significant directivity at all!!
In more than a dozen years of using this set-up, never found any significant directivity...(which is exactly what I expected, from this design / set-up...btw, I've studied/taught antenna system design
for decades, as well as have decades of real-world experience in vhf/uhf repeater systems and antenna design/troubleshooting, etc...)



I used a tri-band (2m, 220mhz, 440mhz) NCG/Comet SBB-224, initially for a tri-band ham HT (but never used it much), and then in 2006 installed a dedicated AIS receiver using that antenna...and it worked great!!
I used the Shakespeare whip with my primary VHF Radio (Icom M-602)...
I assumed when transmitting on the VHF, I desensed the AIS receiver, but never found that to be an issue!

After I installed an AIS transponder (years later) and have both a stern-mounted 3' VHF whip (as secondary antenna) and the Vesper SP-160 VHF/AIS splitter/relay connected to my primary VHF radio and masthead Shakespeare antenna, I've not used the Comet SBB-224 that much, but it does still work...













5) Please read these discussions (they WILL help!):
VHF / AIS Antenna Separation

AIS Problems with Antenna

AIS Problems with Antenna

VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

Vesper AIS SP-160 "relay/splitter" test results, lab/real world




I hope this helps...

Fair winds..

John
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Old 07-06-2017, 16:56   #10
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Re: Multiple antennas at mast head

Quote:
Originally Posted by Auspicious View Post
No. 2m ham is 144 - 148 MHz. Marine VHF is 150 - 160 MHz.

18" is a quarter wavelength (ish). Not good. There will be interaction just by virtue of the presence of the metal in the other antenna. The OP WILL get directivity effects.

It's a poor choice. Put the dual band ham on an upper spreader.
Quote:
Originally Posted by DRS View Post
......... of a 2m dual band vhf/uhf radio and would like the groups advice on the mounting of antennas for both at the mast head. ...................
Hmm... The OP did say VHF/UHF so I am assuming he means VHF & UHF rather than VHF & VHF.

I did get the VHF aspect wrong though, I mis-read and thought he was talking marine band VHF rather than 2m ham band.
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Old 08-06-2017, 07:10   #11
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Re: Multiple antennas at mast head

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Originally Posted by Wotname View Post
Hmm... The OP did say VHF/UHF so I am assuming he means VHF & UHF rather than VHF & VHF.

I did get the VHF aspect wrong though, I mis-read and thought he was talking marine band VHF rather than 2m ham band.
A dual band ham antenna is conventionally 2m and 440. Regardless, it is the metal in the second antenna that is an issue and results in directivity. It's simply a poor idea. Stick a metal rod 18" away from your marine VHF even not connected to anything will cause directivity.
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Old 08-06-2017, 07:19   #12
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Re: Multiple antennas at mast head

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A dual band ham antenna is conventionally 2m and 440. Regardless, it is the metal in the second antenna that is an issue and results in directivity. It's simply a poor idea. Stick a metal rod 18" away from your marine VHF even not connected to anything will cause directivity.
OK, the picture is becoming clearer to me now - must be slow leaner like the teacher said .

So the OP wants to stick a marine VHF antenna and dual ham VHF/UHF antenna on the mast head - somehow I missed the concept

So yeah, not such a great idea after all.

Sorry for the confusion.
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Old 08-06-2017, 07:49   #13
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Re: Multiple antennas at mast head

Personally I wouldn't put all my eggs in one basket. If you loose the mast or get a direct lightening strike up there you'll have nothing. As suggested earlier I'd spread them around the boat.
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Old 08-06-2017, 08:06   #14
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Re: Multiple antennas at mast head

Dave,
I usually agree with 'ya....but here is not the case!
Sorry...

While sticking a metal rod 18" from your VHF marine antenna, in general isn't a good idea....but that is not what the discussion is about...
Quote:
Originally Posted by Auspicious View Post
A dual band ham antenna is conventionally 2m and 440. Regardless, it is the metal in the second antenna that is an issue and results in directivity. It's simply a poor idea. Stick a metal rod 18" away from your marine VHF even not connected to anything will cause directivity.
You see this other antenna (dual-band ham antenna) is not just a stick of metal, but rather is a complex arrangement of loading / feeding that when excited parasitically does not react the way a "stick of metal" would, but even if it was a single-band (2m) antenna, it has feedline attached to it, and the capacity of the feedline significantly effects this antenna's parasitic resonance / response...

As I wrote above this is not something that is usually recommended, as it is not well understood....and also, as I wrote above, a spreader-mounted ham antenna is usually a better option...BUT...
But, this approach does work, and it doesn't present significant directivity!!
(It really doesn't!)


I hope this clarifies things...

Fair winds..

John
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Old 08-06-2017, 08:44   #15
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Re: Multiple antennas at mast head

One thing -- one of our boats had an LED tricolor/anchor light on the mast (from OGM), and multiple antennas.

Since this style of LED light fixture is a singe point of light, we found that each antenna blocked it from some angles. It kind of annoyed me that our anchor light wasn't visible from directly ahead or astern. So keep that in mind if you're putting a lot up there and have this style of light.
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