VHF Antennae (Attn: Bob L)


 

During the last Salish 100 I was disappointed in the range of my AIS receiver. Poking around on the Interweb looking for a solution I found this test of several AIS antenna configurations:

http://www.radioforeveryone.com/p/ais-antennas.html

The same sort of results should be expected for VHF radios. I'm using a 1/2 wave whip antenna for both my radio and AIS receiver. It looks like the greatest improvement comes from adding just one other element, turning the antenna into a simple dipole. (Isn't that what Bob keeps telling us? <g>) I can do that easily, though the element won't be straight.

My 1/2 wave whip is mounted on a metal bracket screwed to a knee for Lazy Jack's hardtop. Would it be OK to fasten the second antenna element __ a copper wire -- to one of the screws for the bracket, then have it bend around the knee to then follow the hardtop stanchion? How should I measure the length, in a straight line from the bottom of the upper element, or along the length of the wire? (see attachment)

Any other ideas?

--
John <jkohnen@boat-links.com>
It s a damn poor mind that can think of only one way to spell a word! (Attributed to Andrew Jackson)


--
This email has been checked for viruses by AVG.
https://www.avg.com


johnacord
 

? A single antenna for both with a splitter??

John A


 

Yes. A Glomex RA201 splitter. You can see where the antenna is mounted here:

https://flic.kr/p/2m73xpS

On 1/17/2022 8:52 AM, john a wrote:
? A single antenna for both with a splitter??
--
John <jkohnen@boat-links.com>
I am glad that I paid so little attention to good advice; had I abided by it I might have been saved from some of my most valuable mistakes. (Edna St. Vincent Millay)
--
This email has been checked for viruses by AVG.
https://www.avg.com


johnacord
 

John,

Is the antenna range  OK for just the VHF radio without the splitter/AIS ?  That antenna should work just fine for AIS.  Perhaps the splitter is not performing well.

When I had AIS I used a separate antenna for the AIS.  Thinking that I did not want anything that could compromise communications especially for the VHF radio.  Had a 1/4 wave with the 4 downward ground radials mounted on a foot or so of pvc pipe mounted to the stern pulpit of the sailboat.  Used that for many years and never had any issues with range or directional performance whether in the open ocean or crowded inland waters (SF Bay area). 

I would be inclined to at least do some testing with the AIS on a separate antenna, or at least on your existing antenna with just the AIS (take along a hand held VHF so not without needed communication).

John A


 

Performance of the VHF radio seemed adequate during the Salish 100, with the splitter, but I never really tested it for range. I'm not worried about the splitter. It defaults to the radio if power to it gives out, or most other things go wrong. High-end radios, like Earl got for Dr. Petra, have built-in splitters, so I guess They're not worried either. <shrug> I have handhels for backup.

The test at the link below compares several simple 1/4 wave antennae. There is a large jump in sensitivity just by adding a vertical element, pointing down, making a simple dipole. Two 45 degree down-pointing elements made a substantial improvement over the dipole, but not as great as the improvement from a single element to a dipole. The antenna with four 45 degree down-pointing elements worked best of all, but the improvement from the antenna with only two down-pointing elements wasn't great.

http://www.radioforeveryone.com/p/ais-antennas.html

I think I can get enough improvement in sensitivity by turning my existing single element antenna into a dipole, and that would be easy to do. I might have enough room along the edge of the hardtop to add two additional 1/4 wavelength elements to the single vertical element, but they would have to be approximately horizontal, not pointing down at 45 degrees.

https://flic.kr/p/2m73xpS

To make a simple dipole from my existing antenna I can easily attach one end of a piece of copper wire to the metal bracket that holds the antenna. But I'd have to bend the wire around a knee for the hardtop so it can follow a stanchion down. I can't pad the stanchion without risking that the side curtains won't fit anymore.

I think anything I do will be an improvement, but I wonder how I should measure the second element to get close to a half wavelength -- along the wire around the bends, or in a straight line. Also, what about that coil the whip screws onto? Will that bugger things up?

https://groups.io/g/oregoncoots/message/56453

I made 1/4 wave dipole, for the AIS, but the only practical place to put mount it vertically is on the aft cabin bulkhead. Pretty low. But I should go somewhere there are AIS transmitters and do some experiments...

On 1/17/2022 5:50 PM, john a wrote:
John,
Is the antenna range  OK for just the VHF radio without the splitter/AIS ?  That antenna should work just fine for AIS.  Perhaps the splitter is not performing well.
When I had AIS I used a separate antenna for the AIS.  Thinking that I did not want anything that could compromise communications especially for the VHF radio.  Had a 1/4 wave with the 4 downward ground radials mounted on a foot or so of pvc pipe mounted to the stern pulpit of the sailboat.  Used that for many years and never had any issues with range or directional performance whether in the open ocean or crowded inland waters (SF Bay area).
I would be inclined to at least do some testing with the AIS on a separate antenna, or at least on your existing antenna with just the AIS (take along a hand held VHF so not without needed communication).
--
John <jkohnen@boat-links.com>
Reality is that which, when you stop believing in it, doesn't go away. (Philip K. Dick)
--
This email has been checked for viruses by AVG.
https://www.avg.com


Bob Larkin
 

Hi Guys -
1-A quarter-wave wire coming out of the coax is basically poor. As you say, John, it needs another half of the antenna, the vertical dipole going down or several "radials" going out horizontally.

2-The half-wave dipole, end fed, is a different deal.  It must have a matching network, usually put into a small tube at the bottom.  It also needs a "ground element" at the base.  BUT, and this is big, for this special case, that ground element can be almost anything.  A quarter wave of wire would be very adequate.  So would a metal mast or even the outside of the feed line.   Because of the very high impedance at the antenna end, almost anything serves as a ground.  It is a very forgiving.

3-The AIS signals are strong, especially from freighters, tankers, and Washington State Ferries---the stuff we are really scared of!  They put the AIS antennas high on the vessels.  I built an AIS dipole antenna into the wall  of the Birdwatcher and also the Scamp.  Both have seemed to be very adequate.

I'm not sure that counting AIS reports at different times is too meaningful.  One might see what the maximum range is on a WS Ferry, or something.

Cheers, Bob


 

Thanks, Bob. I'm glad I finally got your attention. <g> Let me see if I've got this right...

So my existing antenna is an end-fed single half-wave single element. The thingy at the base of the whip is the matching network. Adding a wire connected to the sheath of the coax cable wouldn't turn my antenna into a dipole, but would act as a "ground element", increasing the performance.

So, back to the original questions. Will a wire fastened to the metal bracket, bending around the knee to then follow the hardtop stanchion, work OK? To get the right length, do I measure along the wire length, or in a straight line line? Start measuring at the base of the antenna assembly, where the cable connects?

https://groups.io/g/oregoncoots/message/56453

It sounds like whatever I do with the wire will be an improvement. What about two _horizontal_ quarter-wave wires along the edge of the hardtop?

Where can I find a Washington State ferry around here for tests?

Thanks!

Hope your battery is holding a charge. <g>

On 1/18/2022 12:37 PM, Bob L wrote:
Hi Guys -
1-A quarter-wave wire coming out of the coax is basically poor. As you say, John, it needs another half of the antenna, the vertical dipole going down or several "radials" going out horizontally.
2-The half-wave dipole, end fed, is a different deal.  It must have a matching network, usually put into a small tube at the bottom.  It also needs a "ground element" at the base.  BUT, and this is big, for this special case, that ground element can be almost anything.  A quarter wave of wire would be very adequate.  So would a metal mast or even the outside of the feed line.   Because of the very high impedance at the antenna end, almost anything serves as a ground.  It is a very forgiving.
3-The AIS signals are strong, especially from freighters, tankers, and Washington State Ferries---the stuff we are really scared of!  They put the AIS antennas high on the vessels.  I built an AIS dipole antenna into the wall  of the Birdwatcher and also the Scamp.  Both have seemed to be very adequate.
I'm not sure that counting AIS reports at different times is too meaningful.  One might see what the maximum range is on a WS Ferry, or something.
--
John <jkohnen@boat-links.com>
A belief in a supernatural source of evil is not necessary; men alone are quite capable of every wickedness. (Joseph Conrad)
--
This email has been checked for viruses by AVG.
https://www.avg.com


Bob Larkin
 

Yes, a few quarter wave wires (about 0.46-m or 18-inches, your choice) will be a fine ground for the half wave, and fold/bend to fit.  Don't expect a huge improvement, though.

Washington is always updating its ferry system. They used to have a pretty good looking one in Olympia.  You could buy one, wait for high tide and take it to Elk City, or maybe Mapleton.  Folks could "live" on the upper decks, like a Coot Condo.    BE SURE to test the AIS range first!


Dan
 

Sooooo, some-assembly-required, huh?  If my putative splitter/receiver works; will my 18” ss whip still need a ground plane of some sort?  John and I were speculating on improved vhf performance with a tall, fold-down whip for Walkabout.  I’m supposing that the ground element is in the bottom end of those otherwise fiberglass poles (?).  The part with range from the ferrys, that confounds is how they hide up in places like Rich passage, and spring like a leopard from a leafy tree…dan


Claire Acord
 

Ferries?  More like the ravening wolf coming down on the fold....yikes!
Good antenna info.
thanks
Claire


On Wed, Jan 19, 2022 at 7:40 AM Dan <danashore@...> wrote:

Sooooo, some-assembly-required, huh?  If my putative splitter/receiver works; will my 18” ss whip still need a ground plane of some sort?  John and I were speculating on improved vhf performance with a tall, fold-down whip for Walkabout.  I’m supposing that the ground element is in the bottom end of those otherwise fiberglass poles (?).  The part with range from the ferrys, that confounds is how they hide up in places like Rich passage, and spring like a leopard from a leafy tree…dan


Richard Green
 

I love the visual on that!  The gaping maw of the ferry closing in on a herd of small boats scattering helter skelter for their survival….oh my.

Rich G

On Jan 19, 2022, at 11:57 AM, Claire Acord <whidbeyboatpainter@...> wrote:

Ferries?  More like the ravening wolf coming down on the fold....yikes!
Good antenna info.
thanks
Claire

On Wed, Jan 19, 2022 at 7:40 AM Dan <danashore@...> wrote:

Sooooo, some-assembly-required, huh?  If my putative splitter/receiver works; will my 18” ss whip still need a ground plane of some sort?  John and I were speculating on improved vhf performance with a tall, fold-down whip for Walkabout.  I’m supposing that the ground element is in the bottom end of those otherwise fiberglass poles (?).  The part with range from the ferrys, that confounds is how they hide up in places like Rich passage, and spring like a leopard from a leafy tree…dan





 

A ground element will improve the performance of your little whip.

The long fiberglass antennae are "end-fed" too, so they could benefit from a ground element.

But Bob is the antenna expert...

On 1/19/2022 7:40 AM, Dan wrote:
If my putative splitter/receiver works; will my 18” ss whip still need a ground plane of some sort?  John and I were speculating on improved vhf performance with a tall, fold-down whip for Walkabout.  I’m supposing that the ground element is in the bottom end of those otherwise fiberglass poles (?).  The part with range from the ferrys, that confounds is how they hide up in places like Rich passage, and spring like a leopard from a leafy tree…dan
--
John <jkohnen@boat-links.com>
Love is the delightful interval between meeting a beautiful girl and discovering she looks like a haddock. (John Barrymore)
--
This email has been checked for viruses by AVG.
https://www.avg.com


 

Two 18" horizontal ground elements better than one 36" vertical?

Don't give me ideas... An old WS Ferry would be just the thing for the floating library in Toledo. Plenty of room for books in the upper decks, with Coot Condos as well. The car deck could be closed off to make a dandy boatshop; a veritable enclosed boatyard! :o)

On 1/18/2022 2:22 PM, Bob L wrote:
Yes, a few quarter wave wires (about 0.46-m or 18-inches, your choice) will be a fine ground for the half wave, and fold/bend to fit.  Don't expect a huge improvement, though.
Washington is always updating its ferry system. They used to have a pretty good looking one in Olympia.  You could buy one, wait for high tide and take it to Elk City, or maybe Mapleton.  Folks could "live" on the upper decks, like a Coot Condo.    BE SURE to test the AIS range first!
--
John <jkohnen@boat-links.com>
People who bite the hand that feeds them usually lick the boot that kicks them. (Eric Hoffer)
--
This email has been checked for viruses by AVG.
https://www.avg.com