Using 1/4 wave stainless steel whip at top of mast

[Jammer]

The received wisdom for VHF antennas is to use one of two styles of commercially available antennas:


1) A 1/2 wave base-loaded 36" stainless steel whip, such as a the Galaxy 5215 "Squatty body" - 5215 Classic VHF Squatty Body® Antenna | Shakespeare Marine Antennas


2) A 1/2 wave sleeve dipole in a 48" fiberglass radome, such as the Galaxy 5400-XT. http://shakespeare-ce.com/marine/wp-...xt_5401-XT.pdf



The first of these, though sold as requiring no ground plane, actually uses the bracket, the shield of the coax, and (if conductive) the mast as a sort of counterpoise. I would imagine that these probably work OK in practice but will tend to get a lot of RF inside the boat while transmitting and have an unpredictable pattern. Astute readers will be aware that half-wave end-fed antennas without a counterpoise theoretically don't work at all.



The second option is a much better antenna but its 48" length creates windage, dirty air around the wind instrument, and (as it is not flexible) problems with bridge clearance.


Probably the most commonly used VHF antenna in vehicular applications is the 1/4 wave stainless steel whip with an NMO mount:


https://www.solidsignal.com/pview.as...iABEgID-fD_BwE


While these are short (18") and do flex somewhat, they are also available with an elastomer spring, which reduces the bridge clearance requirement above the mast to somewhere around 4". The spring versions are electrically equivalent. https://www.solidsignal.com/pview.asp?p=bb132r



They do require a ground plane. The NMO mount is a rugged, well sealed through-hole mount. They are designed for blind installation, that is, without tool access to the inside of the surface they're mounted on. Once the mount is in place, the antenna can be removed and replaced simply by unscrewing it, either by hand or with an adjustable wrench. (If you're unfamiliar with the NMO mounts, here's an example: https://www.solidsignal.com/pview.as...iABEgID-fD_BwE)


It is my hypothesis that these 1/4 wave whips, properly installed in the cap at the top of the mast, would make fantastic antennas. I think in terms of pattern and gain they would be better than the "squatty body" types and quite possibly provide the same real-world performance as the 1/2 wave dipoles. The top of the mast makes an imperfect ground plane but I think it would still be sufficient particularly given the presence of the standing rigging and the mast itself. I would use some sort of choke near the top of the feedline to keep the RF where it belongs.


I am wondering if anyone has tried this.


I am thinking of seeing if I can find someone who can sell me the top few feet of a mast that has been scrapped, so that I can run some antenna tests.


I have 60' bridges I must clear to go upstream or downstream of my home port (on the Upper Mississippi). River levels can be a few feet above normal pool and have been all year this year. Bridge clearance is going to be one of the criteria for my next boat and I want to minimize the amount of clearance I give up.

[ka4wja]

Jammer,

Please take note of a correction to your info/idea I'll highlight below....but, first direct to your query:

Originally Posted by Jammer

It is my hypothesis that these 1/4 wave whips, properly installed in the cap at the top of the mast, would make fantastic antennas. I think in terms of pattern and gain they would be better than the "squatty body" types and quite possibly provide the same real-world performance as the 1/2 wave dipoles. The top of the mast makes an imperfect ground plane but I think it would still be sufficient particularly given the presence of the standing rigging and the mast itself. I would use some sort of choke near the top of the feedline to keep the RF where it belongs.

1) I applaud your concern for improving / optimizing antenna patterns for our masthead VHF antennas...and if you read the papers, and examine the models, from W8JI, etc., you'll see that you can improve things with a 1/4-wave ground-plane (with 4 down-sloping radials, and a good feedpoint choke....or an approx 1/2-wave radius flat ground plane!), versus either an end-fed 1/2-wave vertical and/or a sleeve-fed 1/2-wave "coaxial-vertical"...BUT...

But, with the variations / differences being minor compared to the line-of-sight link budgets of Maritime VHF-FM-DSC system, and the fact that sailboat antennas are constantly moving around (in 3 dimensions!), particularly monohulls.... in my opinion, this subject is more like a "solution-in-search-of-a-problem" (and most probably not really a solution at all)....especially since I don't see many sailboat masthead installations that would allow down-sloping radials (45* to 60* down-slope)....yes, you may find some modeling that suggests minor improvements with horizontal radials, but improvements will be slight or immeasurable...

But, an even larger caveat for all of this is that all modeling is of course based on antennas that are rigid and do not move around....and remember that in our applications, even if you make the antenna (and radials) perfectly rigid, the whole antenna is moving around, 'cuz the mast is moving around...

So, while I do wish you well experimenting, I don't have the time to delve into this much more...(and, fyi....please remember that what results you get in a static situation such as at the dock, or that you model on a computer...will be all for not, when out at sea, and/or heeled-over, etc...)


On a vehicle a 1/4-wave whip, mounted dead center of a large metallic roof, is usually better than a 1/2-wave (marginally better)....and usually quite a bit better than a 5/8-wave....(proved by Motorola in the 1950's/60's)
But, because of our mounting/installations on the masthead of a sailboat, not gonna be much difference...

Also, yes, NMO mounts are great!! {fyi, NMO stands for "New MOtorola"..}
Really great on auto rooftops, even at "state-tropper-pursuit" speeds! But, in my opinion, are not needed on sailboats....and to drill/mount and NMO mount on most mastheads would be a really pain-in-the-**s....




2) Onto the corrections / clarifications...

a) A Shakespeare 5215 (3' SS whip) is not a "base-loaded" antenna! And, is not an inferior antenna at all...actually an excellent antenna for a sailboat masthead!

I understand that it is common to think of a "coil" in/on an antenna as a "loading coil", but please note that the base coil of the typical Marine VHF 1/2-wave vertical whip is not a loading coil....and these are not "loaded" antennas....they are full-size 1/2-wave antennas!

This "coil" is part of the impedance-matching network (parallel-tuned network) that matches the very high impedance (~ 1500 ohms) of the end-fed 1/2-wave vertical, to the 50-ohm coax feedline...and hopefully (if well designed / with decent metallic enclosure) chokes off most in-band common-mode currents....
{understand that these are not like the oft-hated end-fed-half-wave HF horizontal wire antennas....certainly not fed the same way! }



b) Further the reason that the "1/2-wave radome verticals" (like the 5400-XT) are 4' long, is because of the 1' long decoupling / feed-section at the bottom of the antenna....

I'm personally not a fan of these "coaxial dipoles" (which is what we used to call 'em), but that's because of poor examples / experiences in the past....no question that the 5400 is a good antenna!!


Also, please understand that even with a 1/2-wave base-fed vertical while a "counterpoise" might be necessary to normalize the pattern (in a model), in our real world at the masthead, the actual masthead of most boats and the rigging of most boats, function surprisingly well as a "counterpoise" for 1/2-wave base-fed verticals...and as such the need for some sort of antenna-base-counterpoise is reduced (eliminated?)....
Also, be aware that with the unfortunate rise of unun-end-fed 1/2-wave HF antennas these days, many are also concerned about common-mode currents on their feedlines flowing back down to the radio shack, but with the well-designed base feed of a single-freq/monoband 1/2-wave vertical antenna the common-mode currents are minimal, and with the masthead and rigging (assuming alum mast) providing plenty of shunting / counterpoise, the concern of common-mode currents on most marine VHF systems is minimal...

So...

So, again, while I do wish you well....in my learned opinion, your concerns of either of these two antennas is more of an intellectual exercise...
Looking at the real-world of our applications and the "why" / "how" they do work well, versus what the average ham / land-based RF engineer would think, or even better than the basic static computer models might suggest...




3) Perhaps I'm incorrect in what I'm inferring here, but it seems as though you are implying that the "3' SS whip" antenna is somehow an inferior antenna, and that they only reason anyone uses one is because of bridge clearance?

As you can see from the details above, this is actually not the case for most of our applications...and while I'm aware that some tout the "1/2-wave vertical dipole" (sleeve-fed 1/2-wave vertical dipole) as a superior antenna....the facts are clear, as long as each is fed properly / appropriately, they are damn close to each other in performance and pattern...any differences are minor compared to the wide variations of the pattern of any antenna on our mastheads, in the real world....


4) As for trying / experimenting with a section of mast??

Originally Posted by Jammer

I am thinking of seeing if I can find someone who can sell me the top few feet of a mast that has been scrapped, so that I can run some antenna tests.

This is an effort in futility....as the antenna pattern is effected by its height above ground, much more than whatever "counterpoise" you wish to design....and this effect is still there up 10-wavelegths high....but does weaken significantly above that, but doesn't stop until the antenna is up 15-wavelengths high...




5) As for bridge clearance....I have the same issue....65' bridge, 63' 8" mast!!

Originally Posted by Jammer

I have 60' bridges I must clear to go upstream or downstream of my home port (on the Upper Mississippi). River levels can be a few feet above normal pool and have been all year this year. Bridge clearance is going to be one of the criteria for my next boat and I want to minimize the amount of clearance I give up.

Use a3' SS whip (like the 5215), and you'll be fine!!

FYI, I scrape my Shakespeare 3' SS whip on the bottom of the bridge at high tide and it touches at mid-tide....clears with less than one foot at low tide (I have 1.5' to 2' of tide)....

I replace the entire antenna every 8 - 10 years, "just because"!
(and keep the old one as a spare in a locker....also have a stern-rail mounted vhf antenna, as ready-to-go-back-up...)



I do hope my comments here are taken in the nice friendly/helpful way I intend....and again, I do applaud your efforts...

Fair winds..

John

[Jammer]

Hi John. Thanks much for your comments.


I am at a loss to understand how the matching coil at the 5215 might work. A schematic would help. From the Shakespeare literature, I understand that the antenna tests as "open" at DC. It doesn't appear to me that a capacitor would be included in the available space, but maybe there is. If you know what's in there, please share, I'd like to understand.


Without a capacitor, and with it reading open at DC, there aren't very many circuit choices other than a series coil.



If, as you suggest, the real-world performance of a quarter wave is going to be roughly the same as either of the commercial antennas, I'd much rather have the quarter wave. Shorter -- better bridge clearance, cleaner air for the windex -- can get a spring to further reduce bridge clearance -- cheaper. I suppose the relative difficulty of installing would vary depending on the mast. Nothing hard about installing a NMO mount while the mast is on the ground for service. The thickness of the mast cap would have to be considered but NMO mounts for thicker surfaces are available and in most cases can still be installed "blind" if there is no access to the other side of the cap.

[capt jgw]

You guys obviously know a lot more about antennas than I do, most of the previous posts were pretty much giberish to me. But I do know that I've had several of the squatty body and the similar Metz antennas, and they work really well on sailboat masts. Once on a trailer boat with the mast on deck, I backed into a tree and bent the whip over 90 deg. Straightened it back out with pliers and it still worked fine! We don't needed to reinvent the wheel here.

[ka4wja]

Jammer,
1) To be blunt about all of this, capt jgw took the words right outa' my mouth...

Originally Posted by capt jgw

We don't needed to reinvent the wheel here.

We really don't needed to reinvent the wheel here...
And, from the beginning I haven't been clear what the problem is that you're trying to solve...

But, just so I'm clear...with 95% - 99% of VHF marine comms on sailboats being line-of-sight (with the link margins we have, that I've discussed in other posts over the years), you can take a 18" piece of wire "coat-hanger" and stick it in the back of your radio and you'd probably be okay in a pinch....and if you stuck it on top of your masthead (as you describe building/installing you 1/4-wave VHF antenna), it would work just fine...(and so does the 3' SS whip vertical, and/or the 4' coaxial-dipole vertical)


2) I'm not clear what the issue is with bridge clearance and windex interference?

If you're mast and windex fit under a bridge, then a 3' SS whip will bend enough to fit as well....if the mast and windex don't fit, it doesn't make any difference what antenna you have up there.
And, why would you need a spring with a short SS whip??

Now while some actual windex's might be difficult to visualize from deck if you have multiple antennas up there, most have electronic wind indicators these days, with masthead transducers....and I can't fathom how a thin whip has much effect on my indicated wind speed/direction? (if it does effect it at all, I really don't care....heck I sailed two years with a broken wind transducer....and barely missed it)

Here are some pics of my masthead at 63' 8" off the water...and the Shakespeare 3' SS whip...no issues with interference nor bending under a bridge...












3) A 1/2-wave vertical (properly-fed and decoupled, which the Shakespeare antennas are) works fine on top of a mast....the pattern differences between them and a 1/4-wave ground plane are minor, a db or so, or less (and that's assuming you provide the 1/4-wave vertical with an adequate ground-plane and decouple it well....if not, you're better off with one of the Shakespeare antennas)





4) As for the coil in the bottom of a 1/2-wave VHF vertical, I don't have the time to write a treatise here (and frankly think delving into this might just stir unnecessary controversy)....and I freely admit that it's been 40 years since I ripped one open....but, just to put a cap on this [pardon the pun]....
From what I recall and from photos I've seen, and most importantly from my understanding of antennas, the coil and its self-capacitance (and/or the capacitance between it and the metallic base) form the parallel-tuned circuit that (at our VHF freqs) transforms the high-imp of the 1/2-wave antenna down to 50-ohms for direct coax feed...it's just that simple..

{please remember that you do not need a "capacitor" in order to have capacitance....at VHF the capacitance needed is small....but fyi, even at HF there are parallel-tuned circuits (traps, etc.) for transmitting antennas that use the self-capacitance of the coil to resonate on some specific HF freq....no "capacitor" needed!!}

So, whatever the internet lore, please just accept that these are not "base-loaded" antennas!

I'm not sure what else to write (you can find dozens of schematics for parallel-tuned circuits and high-imp antenna feed designs on-line....and I can't draw one for you here) ....as I already gave the info on how patterns are formed, and told you who/where you can get more details, etc....
So, I think this should clear things up?


[oh, and if that doesn't convince the masses....aske them why a 3' long antenna (which is a half-wave on 156mhz) needs to be "loaded" anyway?? ]



5) Bottom line:
As I wrote earlier:
This seems like a solution in search of a problem....
And as capt jgw said:
"We don't needed to reinvent the wheel here"!


If you wish to play with antennas, that's fine...
And, as I said earlier, I do wish you well...


Fair winds.


John

[Jammer]

Thanks, John.


As for the problem I'm trying to solve, as noted upthread I have to clear bridges that have a published clearance of 60' at normal pool. I am trying to narrow down the choices for an upcoming move to a larger boat and am considering, among others, a well-known cruising boat with a 57'11" mast height from DWL. It is typical for water levels on the upper Mississippi to slightly exceed pool.


I do not need to clear these bridges every day, only for seasonal relocation etc. away from my present home sailing ground. For these trips I would either remove the wind instrument from the mast or make a mount that allows it to be lowered without climbing the mast. I will not have a tricolor and will relocate the anchor light to leave the top of the mast clear. That leaves the antenna as the highest obstruction. While motoring up or down the river, I do not need a wind instrument but I do need good VHF.


I anticipate using every last inch and will probably set up a video camera and a laser level on a pigstick to verify clearance in some cases.

[Jim Cate]

Jammer, if you are willing to remove wind sensors, etc, for these infrequent journeys, why not remove the VHF antenna too? A temporary one mounted on the pushpit or arch will surely work for river type communications... great DX contacts don't seem important in this scenario.

We have such an antenna as spare/backup and in practice don't notice a great deal of difference in normal usage.

Jim

[Jammer]

Originally Posted by Jim Cate

Jammer, if you are willing to remove wind sensors, etc, for these infrequent journeys, why not remove the VHF antenna too? A temporary one mounted on the pushpit or arch will surely work for river type communications... great DX contacts don't seem important in this scenario.

We have such an antenna as spare/backup and in practice don't notice a great deal of difference in normal usage.

Jim

That is certainly a valid approach. I would have a second VHF setup, with an antenna somewhere other than the mast, anyway, for channel 13.


I've drawn out plans for a retractable wind sensor mount that tilts the wind sensor assembly down below the top of the mast using an air cylinder. Drawing board stuff at this point, no prototype built. Using an air cylinder avoids cluttering the mast with another line and is more reliable and much lighter than an electric actuator, and just needs a tiny 1/8" air line running down the mast. So I may not need a trip up the mast.


Another fact to consider is that portions of the Mississippi do not have cellular coverage, and having a VHF antenna mounted as high as possible improves the chances of getting a response in the event of an emergency. The Mississippi is mostly rural, and the presence of levees, vegetation, and high banks (in the upper Mississippi) make it hard to get any distance on VHF. An antenna mounted on the pulpit is fine for talking to the bridges and other boats.


And there is no intentional HF coverage, though NMG in New Orleans may provide some coverage.


But, yes, I see that I'm in the minority here, and I do agree there are other alternatives.

[smac999]

a mast top is not a ground plane. it's 4" wide. the roof of car is a small ground plain. and works with 1/4 wave antennas if mounted in the middle. ideally a good ground plain would be bigger then the roof of car.

[Jammer]

Originally Posted by smac999

a mast top is not a ground plane. it's 4" wide. the roof of car is a small ground plain. and works with 1/4 wave antennas if mounted in the middle. ideally a good ground plain would be bigger then the roof of car.

A counterpoise for a 1/4 wave antenna should be at least 1/4 wave radius (18") but there are diminishing returns beyond that point. 3 or 4 radials are sufficient. As pointed out upthread there's more to it than that, though, because the surface of the water has an effect, as does the standing rigging.


----


Thanks for all the replies. I have what I need for now. How far I pursue this will depend on the mast height of my next vessel.

[wsmurdoch]

Originally Posted by ka4wja

...As for the coil in the bottom of a 1/2-wave VHF vertical....and I freely admit that it's been 40 years since I ripped one open....but, just to put a cap on this [pardon the pun]....
From what I recall and from photos I've seen...

Here is a photo from Practical Sailor.