DIY marine VHF whip aerial

[electrosys]

Thought these videos might be of interest to the forum, in this age of austerity ...

http://www.youtube.com/watch?v=VjckjeTbmF8
http://www.youtube.com/watch?v=ecp4Ww5pWqs

 

[Rigger Mortice]

He's made a neat job of that.

It makes me realise how expensive things like that are when I get them from a chandler.

 

[Conachair]

Thought these videos might be of interest to the forum, in this age of austerity ...

http://www.youtube.com/watch?v=VjckjeTbmF8
http://www.youtube.com/watch?v=ecp4Ww5pWqs

Ta for that.

Where does the shield go?

 

[electrosys]

Ta for that.

Where does the shield go?

As I understand it (or no doubt someone will correct me ...) it doesn't - the antenna is an end-fed 1/2 wave, not 1/4 wave as stated, so there's no ground plane.

What's puzzling me is that the end of a half-wave is high impedance, and there's normally a tuned cct in the can to match this to 50R coax.

Still - I assume it works, 'as is', so I'll make one up and see ....

... ah - just remembered - I can't. Haven't done the course yet.

 

[VicMallows]

What's puzzling me is that the end of a half-wave is high impedance, and there's normally a tuned cct in the can to match this to 50R coax.

Me too. Do let us know what the SWR turns out to be. For a temporary/emergency antenna I always cut a bit of stiff wire to 1/4 wavelength and mount it somewhere where it can 'see' some sort of ground-plane.

Edit: .... or simply split a length of bell-wire to a half wavelength dipole.....even the downlead bit isn't too bad a match.

Vic

 

[Beadle]

I've not watched the video, but its quite possible to use a matching network to match a half wave vertical. It even gives you a bit of (theoretical) gain.

For an emergency antenna I have always favoured the notion of taking the insulation off 19in coax, rolling the outer conductor back down the co-ax ,running the coax up a spare halliard and using that. Works ok on 2M so no reason why it won't work on Marine radio.

 

[William_H]

I've not watched the video, but its quite possible to use a matching network to match a half wave vertical. It even gives you a bit of (theoretical) gain.

For an emergency antenna I have always favoured the notion of taking the insulation off 19in coax, rolling the outer conductor back down the co-ax ,running the coax up a spare halliard and using that. Works ok on 2M so no reason why it won't work on Marine radio.

+1

That video info is completely wrong and misleading. IMHO. He describes it as a quarter wave antenna whereas his dimension is for half wave length. As electrosys says the end impedance of half wave antenna is very high and where does the outer of the coax go anyway?

I would correct his details by cutting the rod to 17.5 inches or to whatever channel you want to be best at. Then either connect the outer braid to a large ground plane ie steel cabin top or provide a separate ground plane in the form of coax braid down the outside the same length as the radiating rod or several separate wires run from the end of the braid down also the same lenght as the radiating rod.
The ground plane wires become part of the radiating antenna so need to be free and away from other metal like stern rail. So all in all the best construction is to fit it all inside a piece of plastic conduit or even a f/g fishing rod. If the antenna is inside a tube then the most elegant solution is to use the centre conductor of the coax as the radiator. So no joins.
This kind of antenna works really well and I run one of the pushpit with good results.

 

[whipper_snapper]

I made myself one of these out of wire and waterpipe. It works very nicely, it is a bit too high gain for a tippy yot, but perfect for a catamaran, power boat or the roof of my house!

 

[electrosys]

... where does the outer of the coax go anyway?

In an EFHWA (End-Fed Half-Wave Antenna), although desirable, a ground plane is not essential - after all, this is the type of antenna strung out behind Zeppelins in the early days, where - at altitude - ground planes were a tad difficult to come by.

Apparently, it was also a configuration favoured by coast-watchers and resistance fighters in WWII when using improvised equipment - stringing-up a wire into a tree or to the roof of a derelict building, above uncertain ground conditions such as bone-dry earth or rubble.

The advantage of a 1/2 wave over the 1/4 dipole for boaters is, of course, the extra 3db gain in effective radiated power which comes from 'squashing the doughnut', enabling more distant contacts to be made. Not something to be overlooked by those using handhelds which, although many makers often claim 5W max power output for them, this is often only achievable with an external 12/13v supply. With a 7 or 8v battery, some only output 2.5-3 watts.


Later ...

Ah - at last - I think I might have an explanation:

"The central conductor of the coax is attached to the aerial. The braid, which goes nowhere, sees a very high impedance. Well it would, wouldn't it? I mean, it ends in thin air. But, the central conductor of the coax also sees a high impedance at the aerial wire connection. So the end of the coax sees a sort of balanced load."

Not my brain-wave - see: http://www.g4nsj.co.uk/zepp.shtml
from where this possible explanation (together with some editing from me) comes from ...

 

[Beadle]

In an EFHWA (End-Fed Half-Wave Antenna), although desirable, a ground plane is not essential - after all, this is the type of antenna strung out behind Zeppelins in the early days, where - at altitude - ground planes were a tad difficult to come by.

Apparently, it was also a configuration favoured by coast-watchers and resistance fighters in WWII when using improvised equipment - stringing-up a wire into a tree or to the roof of a derelict building, above uncertain ground conditions such as bone-dry earth or rubble.

The advantage of a 1/2 wave over the 1/4 dipole for boaters is, of course, the extra 3db gain in effective radiated power which comes from 'squashing the doughnut', enabling more distant contacts to be made. Not something to be overlooked by those using handhelds which, although many makers often claim 5W max power output for them, this is often only achievable with an external 12/13v supply. With a 7 or 8v battery, some only output 2.5-3 watts.


Later ...

Ah - at last - I think I might have an explanation:

"The central conductor of the coax is attached to the aerial. The braid, which goes nowhere, sees a very high impedance. Well it would, wouldn't it? I mean, it ends in thin air. But, the central conductor of the coax also sees a high impedance at the aerial wire connection. So the end of the coax sees a sort of balanced load."

Not my brain-wave - see: http://www.g4nsj.co.uk/zepp.shtml
from where this possible explanation (together with some editing from me) comes from ...

I suspect the gentleman has a sense of humour.

 

[William_H]

In an EFHWA (End-Fed Half-Wave Antenna), although desirable, a ground plane is not essential - after all, this is the type of antenna strung out behind Zeppelins in the early days, where - at altitude - ground planes were a tad difficult to come by.

Apparently, it was also a configuration favoured by coast-watchers and resistance fighters in WWII when using improvised equipment - stringing-up a wire into a tree or to the roof of a derelict building, above uncertain ground conditions such as bone-dry earth or rubble.

The advantage of a 1/2 wave over the 1/4 dipole for boaters is, of course, the extra 3db gain in effective radiated power which comes from 'squashing the doughnut', enabling more distant contacts to be made. Not something to be overlooked by those using handhelds which, although many makers often claim 5W max power output for them, this is often only achievable with an external 12/13v supply. With a 7 or 8v battery, some only output 2.5-3 watts.


Later ...

Ah - at last - I think I might have an explanation:

"The central conductor of the coax is attached to the aerial. The braid, which goes nowhere, sees a very high impedance. Well it would, wouldn't it? I mean, it ends in thin air. But, the central conductor of the coax also sees a high impedance at the aerial wire connection. So the end of the coax sees a sort of balanced load."

Not my brain-wave - see: http://www.g4nsj.co.uk/zepp.shtml
from where this possible explanation (together with some editing from me) comes from ...

Antennae are complex and often non intuitive to understand. I can't claim to be an expert however....
Antenae for zeppelins and aeroplanes also covert operations in second world war were all high frequency operation. The wire was usually regarded as being shorter than 1/4 wavelength. (or of 3/4 wavelength)
The wire being shorter was then tuned ie adjusted by the use of a variable coil and capacitor for best radiating efficiency. I think you will find that the coupler in doing so also made the ground plane look longer by inductance as well.
In the case of aeroplanes and I imagine zeppelins the wire was would out until the transmitter showed best match. So it would be something like 1/4 wavelength but adjusted to allow that the ground plane (the body of the aircraft) was smaller than ideal. When I first joined aviation radio business trail aerials had almost disappeared. They were both hand wound and electrical but in both cases inevitably the pilot would forget to wind the aerial in before landing leaving it wound around a fence. For HF almost universal in West Oz in those days the fixed antenna nose to tail to wing tip about 27 ft with a coupler became preferred as it still is today.
Now at VHF where an antenna coupler can be used but usually is not. (not in our argument anyway)
If the braid of the coax was left unconnected and the centre conductor connected to a 1/2 wave dipole you would indeed have a balanced load but of no use because the impedance would be so high compared to the 50 ohm cable. power would be reflected back to the transmitter and none goes out.
In practice the outer braid becomes a radiator in some form.
The difficulty is that the crazy antenna described probably worked in terms of being able to talk to people.
It is just that even the smallest of radiated power can give some communications. You need instruments like VSWR meter to confirm that all the power is being radiated. Not much point in having a 25w transmitterr radiating only a few milliwatts of he 25w.
Re the Slim Jim antenna this design can be good for working without a large ground plane. I have built a J antenna which is quite similar and also works well without a ground plane. Or you might say the ground plane is within the antenna. regards olewill

 

[electrosys]

Antenae for zeppelins and aeroplanes also covert operations in second world war were all high frequency operation. The wire was usually regarded as being shorter than 1/4 wavelength. (or of 3/4 wavelength)

In the case of Zeppelins, the key feature was that the radiating element was that of an end-fed 1/2 wave (or multiples of 1/2 waves). The radiating element being fed by a 1/4 wave ladder feed-line. http://www.qikzepp.com/QikZepp_technical_information.html
Hence 'Zepp antenna' has become a synonymous term for the EFHWA.

The J-Pole - which uses similar principles - followed in it's wake.
I believe the originator of the J-Pole was a guy called Lindenblad. I've looked at his 1936 patent (US2131108) - but it's darned difficult to see the simple J-Pole there - Lindenblad's designs were hellishly complex - lots of bits and bobs sticking out all over the place. It's no wonder people have developed a view of antenna design being a 'black art' !

 

[electrosys]

But anyway ... whatever DIY marine antenna is being constructed, you'll need a VHF SWR meter, and the commercial varieties are not cheap (at least they weren't last time I looked ...).

So - "enter stage left" the WideBand SWR Meter by Manfred Mornhinweg (aka 'Homo Ludens'): http://ludens.cl/Electron/swr/swr.html

I built one a few years ago in a die-cast box and it worked fine, but then I gave it away ...

So - this time I think I'll build the 'copper tube' version.


BTW - the 'Ludens' site is well-worth looking around - there are some great pages there !

Colin

 

[SAMYL]

I think by the time you gathered all the components, machined them accordingly and assembled them together, then factored in your hourly rate you will find that one bought from the chandler would be value for money.

And it would be waterproof too, the one described in this video would not last too long in a marine environment.





P.S. Please don't keep knocking the chandleries, they have to price things as recommended by the manufactors. These are tough times so they need our support.
If you don't use them you lose them.

 

[Bilgediver]

Thought these videos might be of interest to the forum, in this age of austerity ...

http://www.youtube.com/watch?v=VjckjeTbmF8
http://www.youtube.com/watch?v=ecp4Ww5pWqs

These give much better perfomance and I have one which does both the marine VHF and ham to acceptable levels. The antenna is sealed inside plumping plastic pipe with end caps and a metal sleeve that is the same length as the short length is fastened on the outside though this is just an enhancement for clamping.

Slim Jim Antenna
http://g0kya.blogspot.com/2009/07/2m-slim-jim-made-from-300-ohm-ribbon.html

 

[electrosys]

I think by the time you gathered all the components, machined them accordingly and assembled them together, then factored in your hourly rate you will find that one bought from the chandler would be value for money.

What - like this one ? (Shakespeare 5206)
http://continuouswave.com/ubb/Forum6/HTML/001272.html

And it would be waterproof too, the one described in this video would not last too long in a marine environment.

The Shakespeare above was waterproof, until ...

I agree about the one in the video - but some attention to the s/s rod base, a cable gland at the co-ax exit, and you could be on the way to achieving a waterproof device. But basically, I like his chosen method of construction. The jury's still out on the electricky however ...

But - if you want to buy, buy. If you like making stuff, make stuff.

Ah - almost forgot to mention - the really nice thing about retirement is: there is no hourly rate.

 

[LadyInBed]

Useful end fed VHF aerial length calculator here + Element length scaling including wire diameter here