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Old 08-06-2017, 09:12   #16
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Re: Multiple antennas at mast head

Vhf antenna on the main, FM antenna on the mizzen. SAT radio antenna on bracket mounted forward off mizzen. All signals connected via Nema 2000 to Raymarine 125.
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Old 08-06-2017, 23:21   #17
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Re: Multiple antennas at mast head

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Originally Posted by Auspicious View Post
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?
All the new digital TV channels in Australia are included in a fairly narrow band width in the UHF frequencies. Being a suck it and see sort of bloke I calculate the length required for the centre frequency and soldered a bunch of red bean cans together for a good OD length ratio and mounted it in water downpipe at the top of the mast. Looks a bit bulky but works well.
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Old 09-06-2017, 06:16   #18
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Re: Multiple antennas at mast head

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Originally Posted by ka4wja View Post
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...
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Originally Posted by ka4wja View Post
Dave,
I usually agree with 'ya....but here is not the case!
Sorry...

John,

Always good to talk with you.

Anything I don't raise you can safely assume we agree on.

> 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...

If you are designing an antenna intending it to be directive than resonance and impenance are important. Hunks of metal, even a collinear dual-band or multi-band antenna, will interact with other antennas.

I've demonstrated this twice. The easy, no surprises one was two Metz marine VHF antennas at the masthead, one for voice and one for AIS. The impetus was a dockside discussion about interaction, much like this one. *grin* I didn't have a spectrum analyzer but I did have a SeaTow automated radio check. Driving the boat on courses 30°apart and calling the radio check robot showed directivity forward and nulls off the sides. Taking the AIS antenna down made the VHF omnidirectional within our ability to hear differences.

The second experiment, different boat, was with a Shakespeare VHF (I don't remember which one) and a Diamond X-50A dual-band antenna. With the same methodology showed directivity aft with discernable reductions forward and to the sides but no identifiable nulls per se. Moving the Diamond to a spreader returned the VHF to omnidirectional. Measurement of the Diamond performance on the spreader was a little harder (we were dependent on random people on a repeater) but had some minor directivity toward the shroud on the mount side.

My experience suggest that there is noticeable interaction that results in some directivity. The question left unanswered is if the impact is small enough as to be negligible in all conditions. I gather you suggest that is the case. I just don't know. I do know that you (big you - everyone) doesn't know about the signals you don't hear.



In my opinion given the facts in evidence including antenna theory (which you and I both accept as fact) that the masthead of a sailboat is a pretty small place to mount multiple antennas without accepting some interaction that may or may not be significant. I don't recommend it.



73 es sail fast de dave KO4MI
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Old 09-06-2017, 06:20   #19
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Re: Multiple antennas at mast head

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All the new digital TV channels in Australia are included in a fairly narrow band width in the UHF frequencies.
Really? Isn't it 500 - 820 MHz? That's a whole lot of spectrum.
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Old 09-06-2017, 07:46   #20
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Re: Multiple antennas at mast head

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. . .

And to repeat, height trumps power (including ERP and EIRP).

This is a very good post, but I can't agree with the quoted proposition.

Height and power address different issues, so neither actually trumps the other.

If you have line of sight -- say you're talking to a ship with VHF antenna 45 meters above the water -- a few meters more or less of height of your own antenna won't make any difference except maybe at extreme ranges when you are ALMOST over the horizon from his antenna.

Power might not make much difference in any case -- as long as you have a good antenna, well installed. 25 watts is way overkill for most marine VHF contacts between most stations.

But they are really not substitutes for one another.

In my opinion it's better to give priority to the most important use -- VHF DSC and voice communications, which are by far the most important thing you are using radio for, and might be a matter of life and death someday. The other stuff can live with being on a spreader somewhere. In my case, the first spreader is 10 meters above the water, so plenty high enough. Just the mast and rigging create some directionality, but for fun stuff (ham VHF/UHF) and even for AIS, I think it's worth putting up with that, in order to avoid compromising the main use.
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Old 09-06-2017, 23:09   #21
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Re: Multiple antennas at mast head

WOW thanks to everyone for the input. The mast went back up today with just the marine VHF on top. As I have yet to buy the dual band radio antenna placement will wait till then.

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Old 10-06-2017, 06:36   #22
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Re: Multiple antennas at mast head

Dave,
Good to talk to you, too!

And, yes...anything metallic in an antenna's near field will have an effect on it and its radiation pattern!
How much effect is subject to way too many variables to make any hard 'n fast statements...except that if there isn't anything nearby, then you don't have any problems...

While I do stand behind my words that this can be done and not be detrimental to your primary VHF antenna's pattern....I think I also was clear to Derrick that "a spreader-mounted antenna would be a batter choice"...


As for my specific installation...
Exact figures from memory (from a dozen years ago) have faded....but here's the best I can recall...

In 2004 looked at the SBB-224 and the LMR-240uf, looked at the capacitance spec on the coax, ran some numbers and found it would be parasitically resonant down in the air band, and figured that would fine...
Installed new antennas (both Shakespeare and the 224) and ran new cables for both...
Everything worked fine, had great VHF comms, no issues....

Suffered direct lightning strike in 2006...both antennas at masthead destroyed...replaced all mast wiring (as well as all NMEA and electronics wiring) and did a complete electronics upgrade...
When commissioning new gear, got coaxes mixed up (didn't know which went to which antenna!), so had to go up mast....removed the whips from both and did some tests...had my sister at Nav Station listening to couple of the weaker / fringe NOAA weather stations (I get 3 perfectly clear and another 3 - 4 that vary day-to-day and are good indications), and watching my power/VSWR meter, with me at masthead swapping-out the whips....
Found out which antenna was which (surprisingly the SBB-224 did well on VHF Marine, a slightly higher VSWR and a little weaker receive, but it shows that even a "coat hanger" at 65' works!!)

By trial-n-error got it all figured out in less than 15 minutes, and in the process found no difference in VHF Marine / NOAA Weather reception from my Shakespeare antenna, whether the SBB-224 was mounted on masthead or sitting in my lap on the bosun's chair 5' below the masthead...

Now, this isn't in anyway a scientific study!!
This was just at dockside, with the few different NOAA weather stations (most of the beams, and one off the aft)
And, when I wrote earlier that I found no significant effect on directivity, it was both the past dozen years using this set-up and these "tests"....
So, while I'm perfectly happy with my results, I am also happy to add this info here....and reiterate my caveat that "a spreader-mounted antenna is probably a better approach for most"!!



We have different results, but also did different tests...
And, we each have slightly different opinions here...what I accept as "not significant" you may have seen as significant, etc...
Quote:
Originally Posted by Auspicious View Post
The question left unanswered is if the impact is small enough as to be negligible in all conditions. I gather you suggest that is the case. I just don't know. I do know that you (big you - everyone) doesn't know about the signals you don't hear.

In my opinion given the facts in evidence including antenna theory (which you and I both accept as fact) that the masthead of a sailboat is a pretty small place to mount multiple antennas without accepting some interaction that may or may not be significant. I don't recommend it.
I'm not recommending close-spacing of vhf antennas at the masthead...just saying that if you (the big "you") accept some interaction, and are using different antennas / different resonances, the effects are not significant...(at least in my opinion)...
But, if you're mounted 2 similar / duplicate antennas, horizontal spacing should be 3' absolute minimum and 6' is recommended minimum....but preferably separated vertically (one above the other)...


Hope this helps clarify...

Fair winds..

John


P.S.
On a side note, on my repeater tower....at ~ 1100' above ground on a 1368' tower...I've got two, triband antennas....mounted on a stand-off gate, about 9' - 10' off the side of this massive tower!!
I have 146mhz/222mhz/444mhz on one antenna....and the other antenna, hung immediately (directly) below it by only 16", has 52mhz/146mhz/444mhz....
VHF isolation is on the order of 45db (I'd have to find my notes to be exact, but it's between 42 and 45db)....
These are modified ham antennas, that are usually not thought of as being adequate for repeater use...but have replaced them only once in 20 years (after a wicked storm) and then "just because", not because they quit working!
The flexible coax on the gate is RG-214 (with PL-259's) and the feedline all the rest of the way to the equipment racks is Andrew LDF-5-50A (two runs of ~1200' of 7/8" Heliax....frigging pricey transmission line!)
KA4WJA/R, 146.97, Voted "Best" (non-linked) repeater in "4" Call Area...
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Old 10-06-2017, 06:58   #23
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Re: Multiple antennas at mast head

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Really? Isn't it 500 - 820 MHz? That's a whole lot of spectrum.
As I said I'm a suck it and see sort of bloke and I sucked it and get to see a lot of TV along about 1,000nm of the coast line I regularly cruise.

There is a phenomena in management called "paralysis by analysis" and quiet often it's better just to go ahead and try something out rather than get involved in an extensive design process - I think it's called the empirical process.
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Old 10-06-2017, 07:34   #24
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Re: Multiple antennas at mast head

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Originally Posted by ka4wja View Post
And, yes...anything metallic in an antenna's near field will have an effect on it and its radiation pattern!
Agreed. There is no substitute for vertical separation, even just a little like your repeater example, or frequency separation. For most marine applications, voice VHF, AIS, and NOAA weather channels might as well be the same frequency.

I would say that you can put multiple antennas at the masthead but you shouldn't. If you have to for some really good reason ("it's hard to run coax to a spreader" is not a good reason) you should do some testing at different aspect angles to see what you really ended up with.

John or I (or both) can write you a test protocol that is heaps better than the "let's see" testing we each did.

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As I said I'm a suck it and see sort of bloke and I sucked it and get to see a lot of TV along about 1,000nm of the coast line I regularly cruise.
I was by no means poking at your solution. I was poking at your vocabulary. *grin* That's a lot of spectrum.
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Old 10-06-2017, 08:03   #25
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Re: Multiple antennas at mast head

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Agreed.

I was by no means poking at your solution. I was poking at your vocabulary. *grin* That's a lot of spectrum.
I suspect I just calculated it for a frequency of 200mhz.

The last one I built was made using 1" thin walled aluminium tube and probably won't be as good on the high and low channels since it doesn't have the more favourable length diameter relationship.

Australian TV uses a fairly narrow part of the uhf spectrum at the lower end.
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Old 10-06-2017, 16:17   #26
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Re: Multiple antennas at mast head

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.............

Australian TV uses a fairly narrow part of the uhf spectrum at the lower end.
Yes, in regional areas.
Capital cities still use VHF - IIRC.
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