VHF antenna: short or (end-fed) dipole?

[masthugg]

I am looking for some advice or theory on VHF antennas. SInce I am fluent in microwave circuits and have a basic (but almost forgotten) theoretical understanding of antennas, I recognise that most of the stuff on antennas on boat forums is superstition, "some-guy-said", "high gain is always good", or anecdotal experience with only one antenna type.

Surveying the market, I recognise four different antenna types:

1. Steel whip (~1 m, half wavelength)
2. fibre glass pole (~1 m, half wavelength)
3. short antennas (~2-3 dm, "short "dipole")
4. 5/8-wave antennas.

Let's focus on types 1-3. There are a lot of info on half-wave dipole antennas online, but it is mostly about center-fed dipoles, and I am assuming that 1-3 are generally end-fed? Let that be the first question: Are these antennas end-fed? As far as I understand, a center-fed antenna would mean the feed line has to extend approx 90 degrees from the dipole to not interfere with radiation pattern and impedance. That's why I think most marine antennas are end-fed.

Second question: many antennas say "built-in groundplane". Silly phrasing, I would rather say that a ground plane isn't needed. But how does the mounting affect anntennas 1-3? Let's consider thre conditions: masthead (metal), pulpit (metal), fibre glass roof.

Third question: what do you recommend for a sailing vessel with a 9 m (27 ft) mast? I would like a mast mount later, but will start with pulpit mount since the boat is in the water already.

Regarding the third question, it is my impression that a short dipole and a half-wave dipole has similar properties, except that the short dipole presents a nasty impedance. To what degree is that matched and taken care of in the off-the-shelf-antennas? Since the range is anyway mostly limited by the earth's curvature, I have a hard time seeing when i would care about anything else than severe mismath or a really skewed radiation pattern.

Please feel free to answer only a fraction of the questions above

 

[ShinyShoe]

I've come across (broken) fibreglass whips that appear to be centre fed - i.e. there is a narrow shielded wire inside the whip that goes the middle of the antenna

27ft mast will make a big difference to range vs a water level one.

Beware the effect of heeling!

 

[lw395]

There are many ways of matching short antennas.
Series coils, transformers, tapping part way along the radiating bit.
Many short antennas use the mounting or coax feed outer as a 'counterpoise' which you can think of as somewhere between a ground mass and the other half of a dipole.
A good consumer antenna will give an acceptable match with a mast, pushpit, or pole as the counterpoise.
I think a short pole on the pushpit is a reasonable compromise if you can't reasonably fit a masthead antenna.
Mounting directly on the pushpit can be vulnerable and I've seen some lashups where somebody board the boat could be at risk of a poke in the eye from the aerial.
Worry more about getting the coax connection right and keeping the inside of the coax dry.

 

[MM5AHO]

Most the whip type antennas are not dipoles, they are a "quarter wave vertical".
By definition a DI pole, is two. Two quarter waves, one going up the other going down, fed where they meet.
There are dipoles in marine use, as mentioned above, inside a fibreglass tube and centre fed. There have slightly more "gain" that the standard quarterwave.
But gain isn't everything in this situation. Gain is a bit like sensitivity (on receive) and like an amplifier on transmit. How an antenna does that is to concentrate the signal in a similar way to how a reflector does in a torch. The same torch bulb appears brighter with a reflector that the naked bulb. But its the same bulb. Just that the light that would otherwise have gone upward, downward, and backward, now gets reflected in one direction.
In a vertical antenna, the signal that would otherwise have gone straight up and straight down (useless) gets "reflected" out to the horizon.
Why this is of limited value, is that it goes mostly out at 90 degrees to the vertical. But now sail, and get a 30 degree heel on, and your vertical is not longer vertical. So the concentrated "beam" is now going to windward at 30 degrees above the horizon, and downwind down into the sea (30 degrees below the horizon). A gimballed antenna would solve this issue, but no-one does that (why not).
So, a quaterwave is a good compromise between concentrating the signal to the horizon, but not too much. A dipole would do that more-so, and some other antennas even more so. And the more you do that, the less use the antenna is on a yacht that heels.
So the dipole much more common on big ships (less heel).
A 5/8 wave antenna is something in the middle between dipole and 1/4 wave.
Strictly the 1/4wave does need a groundplane, but works OK without. The sea being underneath helps, though it's a bit far away (9-10m below) at these frequencies.

 

[William_H]

Most the whip type antennas are not dipoles, they are a "quarter wave vertical".

So, a quaterwave is a good compromise between concentrating the signal to the horizon, but not too much. A dipole would do that more-so, and some other antennas even more so. And the more you do that, the less use the antenna is on a yacht that heels.

Strictly the 1/4wave does need a groundplane, but works OK without. The sea being underneath helps, though it's a bit far away (9-10m below) at these frequencies.

I would disagree that a quarter wavelength whip works OK without a ground plane. I guess more from theory than experience because of course anything will work a bit. Anyway a metal mast makes a fine ground plane for a quarter wave whip. A mast top whip will work well due to height above water.
I use a quarter wave whip on my stern rail. It actually sits in an extended corner vertical ss post so base of the whip sits about 50 cms above the top rail. Makes itless vulnerable to being damaged or hurting people. It seems to have acceptable
VSRand provides adequate range for my needs.
I used some ss rod which was flattened at the bottom then drilled to take a screw and lug onto the coax centre. The coax outer was connected via a lug to the top of the post. I jambed some plastic conduit in the top of the post with a slit dwon one side for the screen wire to exit. More plastic tubing and a lot of glue gives support to the whip up to about 70mm above the top of the post. It has been in use for some years very successfully.
I have also made and used a so called coaxial antenna. Here there is a quarter wave whip connected to the end of the coax centre. The braid connects to a sheath of additional braid of quarter wave length on the outside of the coax. This latter must be insulated from the feed coax outer except at the feed point. The additional braid makes a fine ground plane the whole can be fitted inside a fishing rod or similar.
At home I have a "J" antenna which seems to work well also has built in ground plane but I am a bit confused on how it works or which part is the ground plane. http://www.hamuniverse.com/jpole.html
good luck olewill

 

[Skylark]

I would keep it simple, personally. The vast majority of sailing vessels seem to use quarter wave stub antenna at the top of the mast. Height is likely to give more gain than a fancy antenna.

Modern radio sets do not like high VSR and will reduce power to safeguard against failure of the power amp stage.

By definition, the half wave dipole is known as the "fundamental" antenna. It should be "balanced" in other words two identical halves fed in the middle. It should also resonate at the same frequency as being transmitted. Simple formula of frequency, length and velocity of propagation constant.

Now in practice, everything starts to be a compromise. The other half of the quarter wave stub is commonly earth, ground plan or counterpoise. Coax cable is unbalanced and a balun transformer should be installed between coax and dipole.

Also need to think about losses. High grade coax will serve you better than cheap stuff, well worth the additional cost.

It's a fairly safe assumption that a commercially available, commonly used, marine vhf antenna will work.

Final check, measure VSR across the range of transmitted frequencies and if it's less than 1.5:1 it is good.

My knowledge of vhf is based upon 1982 learning so if anyone cares to contradict me, claiming that I'm talking bolox, feel free!

 

[MM5AHO]

"I would disagree that a quarter wavelength whip works OK without a ground plane." (OleWill)

I'd have better worded that by saying it works OK without a specially installed groundplane like terrestrial verticals often have.

 

[rob2]

Interesting though it is to read the discussions about various DIY antenna (and yes, I've made a few) most of the discussion doesn't address the issue of suitability of various designs. I'm not the one with the theoretical knowledge to address this, but I recall being told that the higher the gain of an antenna, the more limited is the vertical element of its propogation. This means that typically a sailing boat will use a low gain design so that it wil;l perform acceptably when heeled. whereas a motorboat can use a higher gain version as it will remain more upright.

I made a few emergency antenna, using the Slim Jim design. Making it out of the parallel feeder cable means it can either be encased in small conduit or rolled up for storage. But it's not a design that lends itself to permanent mounting on a yacht!

Rob.

 

[lw395]

A higher gain antenna does indeed have a smaller beam width.
Think of it as focussing its radiation into a smaller angle or part of the sphere.
Mostly we are talking about 'omni-azimuth' antennas so only the vertical beam width is up for discussion.

But some antennas of even low gain can be putting most of their energy in the 'wrong' direction. Short antennas close to a ground plane being prone to squinting say 30 or 40 degrees upwards for example. Handy for talking to a satellite, bit poor for targets on the horizon.

All good fun.