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For marine VHF (156~162 MHz), the most commonly used antenna is a 1/2-wave dipole or a collinear array in the form of a fibreglass rod. Collinear arrays are used to increase gain in some antennas. Dipoles/collinear arrays by their nature do not need a groundplane. This much I gather.

However, another popular type - especially with the sailing crowd - is the stainless steel whip antenna. Also supposedly a 1/2-wave antenna, it is roughly 1 m long.

The following page has examples of both types of antennas - AV7M is a fibreglass rod dipole, and AV53BIS3 is a stainless steel whip: http://www.comrod.com/category.php?categoryID=129

Further examples of stainless steel whips specifically:

According to the datasheet and other information available on this type of antenna, the stainless steel whip is an end-fed dipole, and does not need any external ground plane.

The following page describes the concept of an end-fed dipole: http://www.aa5tb.com/efha.html The cylindrical base of the VHF whip then, it can be assumed, contains the LC matching circuit/balun(?) described.

We learn that the antenna doesn't work without a "counterpoise" - and it seems reasonable to assume that the VHF whip is in fact constructed similarly to Figure 15.

Further reading-up on counterpoises brings much confusion about their nature: http://www.antennex.com/shack/Dec06/cps.html

With this in mind:

  • While several of the whip antennas referenced above are described as DC-open, whereas "Figure 15" is DC-shorted, is it possible that they are of a "Figure 15"-similar design? If not, then what are they?
  • What is the nature of the (probably misnamed) "counterpoise" in "Figure 15" et al (which then, presumably, consists of the antenna's cylindrical brass base, feedline, and the radio equipment)? What is its role in allowing the standing wave/electrons-on-the-move in the radiating element to excite the E-field and, well, radiate?

Would also much appreciate references to literature that explains the physics of antennas in general and these types in particular, with sound scientific base without going deeply into the maths. Have had a hard time finding quality literature among the seemingly vast quantities of "black magic" antenna cookbooks.

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  • $\begingroup$ I looked a bit at that AV53BIS3 and couldn't find enough information for me to really say for sure how it works besides guessing. Do you maybe have a few more examples of similar antennas? $\endgroup$
    – Phil Frost - W8II
    Commented Sep 17, 2015 at 17:00
  • $\begingroup$ @phil-frost One: scan-antenna.com/product/vhf23 - Two: glomex.it/shop/prodotti/diporto-antenne-marine/antenne-vhf/… - Three: pacificaerials.co.nz/Marine/… - Four: shakespeare-ce.com/marine/wp-content/uploads/sites/4/2015/04/… - The last one says it's an end-fed 1/2-wave design. But it also says it's DC-open circuit (as does the one in the previous link, from Pacific). Then it cannot be a "Figure 15" design, as then it would be DC-shorted. I'm confused. $\endgroup$
    – user5314
    Commented Sep 19, 2015 at 7:29
  • $\begingroup$ Yeah, me too. I can only guess that there's some kind of matching network in the base to accommodate the very high impedance at the end, but that still leaves the question of what they do about common-mode currents on the feedline. I'd think trying to end-feed a dipole at its highest impedance point would make it hard to get right. Maybe they do nothing and that's good enough. But that's all just guessing...hope you get a good answer from someone, this is a good question! $\endgroup$
    – Phil Frost - W8II
    Commented Sep 19, 2015 at 17:57
  • $\begingroup$ @PhilFrost More research suggests that they are "base coil-loaded" antennas: link 1 (scroll down to "Loaded Coil Antennas", link 2. So, rather than a "Figure 15" transformer design, they just use a series coil. Why? This link suggests the impedance at a 1/2-wave end-fed is 1000:s of ohms resistive. How does a coil then help? $\endgroup$
    – user5314
    Commented Sep 21, 2015 at 1:41
  • $\begingroup$ Also, since these antennas don't need ground planes - what about the screen of the feedline? It would seem that in a base-coil-loaded whip, the feed line screen is just connected to the metal cylinder at the antenna base, which has no obvious relationship to the wavelength or anything. Is this, then, a "counterpoise"? If so, then what is a counterpoise, speaking in terms of physics? (c.f. the link on counterpoises in my original question) - or am I on the wrong track? $\endgroup$
    – user5314
    Commented Sep 21, 2015 at 1:50

2 Answers 2

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Without having one of these antennas to disassemble (perhaps destructively), I can't tell you exactly how they are constructed. But maybe I can address some of your underlying concerns.

Firstly, counterpoise. In one sense, this is an elevated screen of wires designed to take the place of Earth. This sense developed with the Marconi antenna (what we'd probably call a "vertical") in the late 19th century.

In the other sense, counterpoise is the "other half" of the antenna. If charge is being removed from the antenna, then it is being added to the counterpoise, and vice versa. This must be so, due to the law of charge conservation. In this sense, the counterpoise may be the Earth, or radials, or the other half of the dipole, or the feedline, mast, tower, or whatever else may be connected or capacitively coupled to the antenna system.

Some people will tell you that one of these senses is wrong, but the fact is that "counterpoise" has no rigorous definition: you have to figure it out by context.

Now the trouble with end-fed dipoles is this: if you are putting charge into the antenna from the end, where are you getting the charge from? In a vertical we can take it from the ground and put it into the antenna, and in a center-fed dipole we take it from one half and put it into the other. But with an end-fed dipole, there's no "other" thing: there is no counterpoise.

In practice, the feedline or mast will become the counterpoise. Since you probably didn't intend for the feedline to have RF current all over it, you might want to do something about that. W8JI has a pretty good article on the subject. In summary, you may want to isolate the feedline with a transformer, but if you don't, it's not the end of the world. 25W transmit power probably isn't enough to cause arcing or RF burns no matter what you do. Without disassembling an antenna it's hard to say exactly what the feed and matching arrangement is.

However, it is pretty safe to assume that in all cases, the feedpoint impedance is high, and we need some way to make it lower to match 50 ohms. If the impedance is already purely resistive, then a transformer with the right turns ratio will do the trick.

But we can also accomplish a step-down in impedance with either:

  • a parallel inductor + a series capacitor, or
  • a series inductor + a parallel capacitor.

schematic

simulate this circuit – Schematic created using CircuitLab

It's easy to see how this works on a Smith chart: if we start at 1000 ohms (the green dot) then there are two ways we can get to 50 ohms (the center):

smith chart

Additionally, making the antenna a little too long, or a little too short, will introduce a reactive component to the feedpoint impedance. So it very well may be that a clever antenna designer can use this to take place of one of the components, and then achieve an acceptable match with just one other component.

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  • $\begingroup$ I think the benefit of the "end fed" is that it can be mounted to a fibreglass boat without special grounding arrangements. I agree the coax screen is the ground, but because the Zfeed is high, the voltage is mostly developed on the hundreds of ohms end-fed halfwave resonant wire, not so much on the ~70 ohms random feedline. Using a bit of leftover inductance is exactly what I have done too; it's almost easier than not doing it. You simply put in the calculated capacitor, and then tune the antenna for best VSWR. $\endgroup$
    – tomnexus
    Commented Sep 21, 2015 at 20:41
  • $\begingroup$ Great write-up, thanks. A few follow-up questions, one per comment/post: I'm fairly confident, after what I've read online, that the cylinder at the base of these antennas just contain a series coil to the center conductor/half-wave element. I'll continue from that assumption. Your "other sense" of the meaning of counterpoise matches my intuitive feeling that it's "just a source/sink for charge bearers". Then, it could take any form/shape, right? Why, then, are they almost always 2-dimensional in the ham world? (TBC >>>) $\endgroup$
    – user5314
    Commented Sep 28, 2015 at 9:54
  • $\begingroup$ Now, looking at this graph of impedance vs. length-frequency ratio, keeping in mind also the I, U distribution over a dipole of "any" length, I get the impression that the coil really only extends the antenna electrically by 1/4 wavelength to put the feedpoint at a U node/I max, and a parallel capacitor provides the capacitive part that "the ether magically gives to the 1/2-wave element"? In the marine whip, can this capacitance simply be that between the coil & the cylinder? $\endgroup$
    – user5314
    Commented Sep 28, 2015 at 9:54
  • $\begingroup$ Now, if my previous understanding is correct, that an extra 1/4 wavelength is just coiled up with some capacitance sprinkled on top, then what is the advantage of coiling it up in the first place? It would seem to be simpler to just end-feed a 3/4-wave element, as it is. Is there some basic flaw in my reasoning? I would naively think that the 3/4-wave element would also put more power into the "ether". Is this about the radiation pattern? Thanks for helping a sailor-turned-RF noob navigate these strange waters... $\endgroup$
    – user5314
    Commented Sep 28, 2015 at 9:55
  • $\begingroup$ @maxxflow that's quite a lot of follow-up for the comments. Maybe ask a new question(s) and link them here? $\endgroup$
    – Phil Frost - W8II
    Commented Sep 28, 2015 at 13:15
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Your posting is a good reminder how prevalent end-fed half-wave (EFHW) dipoles are in the commercial antenna biz. The marine VHF antenna industry seems to be the most obvious of the bunch given the need for an antenna to function "well" without benefit of a conductive structure to mount upon.

My ham VHF/UHF mobile antenna is another EFHW example where measurements confirm improvement over its predecessor.

Looking into the various EFHW offerings reveals an exemplar matching transformer at the base that looks something like this...

The end-fed half-wave dipole antenna with magnified view of transformer.

More details available in this article. The above operates well, in simulation, without any need of the oft mentioned counterpoise, etc. Measurements of an HF analog to the above reinforce the ability of an EFHW to operate independent of additional conductors albeit with a bit of loss in the slightly different transformer structure.

Difference between EFHW transformer configurations

However, feedlines are the norm so I am exploring the effects there as well.

The following graph details the behavior of the VHF/UHF mobile EFHW above against varying lengths of feedline with a grounded end/radio. The frequency is 146 MHz.

The peculiar 13.5′ feedline length of the Diamond K515 mount.

Clearly there are certain lengths one should avoid lest we rob energy from the antenna, but the good news is most lengths seem to play well enough. One can only wonder if the unusually specific length of 13.5 feet was chosen by the maker of my mobile antenna to precisely avoid certain lengths that will entice current from the antenna down the feedline. More detail in the article. This assertion is pure conjecture on my part, but I do wonder if the stock feedline lengths of the marine VHF mobile antennas are also of some specific and peculiar length.

In summary, the longevity of the EFHW design in the world of radio, especially VHF marine, gives comfort the manufacturers know what they are doing and the notion feeding a half-wave dipole from the end = NBD.

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