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Old 14-07-2014, 03:14   #16
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Re: VHF Radio - Tricks

Quote:
Originally Posted by Snowpetrel View Post
Anybody got the proper length to make your own vhf antenna. I saw instructions once in a vhf manual. Sounded like you peeled the coax away a set length to expose the core, and did some Thing with the cover? Cant quite remember what. But that would be a handy trick if it worked ok.
You want a quarter wavelength on inner and a quarter wavelength on the outer shield. Hold the antenna in the vertical plane; The inner is normally "up" and the outer (shield) will therefore be "down". The resultant antenna is what is known as a 1/2 wave vertical dipole.

The formula for finding the wavelength is
300,000,000 = wavelength (in metres) x frequency (in Hertz).

I'm sure you can do the maths .

FWIW, 300,000,000 is the speed of light in metres per second.
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Old 14-07-2014, 06:15   #17
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Re: VHF Radio - Tricks

What Wotname is suggesting can work. A bit more detail might help visualize. What you want to end up with is an exposed section of the center conductor at the end. The original shield that was around the center is folded back down around the outside of the coax. The dimension given for the upper exposed center conductor is correct. However you will find that the VSWR is better if the exposed outer that is folded back is about 80% of the length of the exposed center. The reason is that the black jacket of the coax has a velocity factor of about 0.8. That just means the speed of light is slower by 0.8 inside the black plastic. The folded back shield should be as tight against the black cover as you can make it. Wrapping with black electrical tape can help. Make the center conductor a couple cm (1 inch) longer and make a loop. Tie a piece of small stuff to the loop and use this with a halyard to raise the homemade antenna. If you get the lengths right it will transmit/receive as well as most commercial antennas you can buy but it will not be as rugged. Even if you don't get the lengths just right it will work surprisingly well when hoisted near the top of the mast.

For channel 16 (156.8MHz) the upper exposed center is about 18.75 inches long (to the tip of any lanyard loop) and the folded back outer conductor is about 15 inches.
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Old 14-07-2014, 06:30   #18
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Re: VHF Radio - Tricks

Good info. What's the informed thinking on antenna location on the mast? I'm a little gun shy, as the lightning bolt that took all our electronics out seems to have started with the VHF antenna on the top of the mast. This boat came with one of those fuzzy lightning things up there, too. The metal VHF whip was sticking up higher than the fuzzy lightning thing. The lightning seems to have liked that, and I assume it used the braid of the coax to start it's very brief but thorough round of mischief as it dove into the sea through our boat.

I've been thinking of mounting a VHF antenna upside down, under the radar antenna mount. This would put it thirty feet above the water, but it would be shielded by the mast behind it. So, I wouldn't be able to talk to people behind me as well. This is inconvenient, but not a show stopper. Rarely do people behind me try to call. And I would expect a reduced range aft, not completly blanked. I could alter course long enough for a clear signal path if needed. Are these correct assumptions?

It's seeming that putting the VHF whip the highest thing on the mast is good for beam pattern, but does it attract lightning?
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Old 14-07-2014, 06:53   #19
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Re: VHF Radio - Tricks

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Originally Posted by Wotname View Post
You want a quarter wavelength on inner and a quarter wavelength on the outer shield. Hold the antenna in the vertical plane; The inner is normally "up" and the outer (shield) will therefore be "down". The resultant antenna is what is known as a 1/2 wave vertical dipole.

The formula for finding the wavelength is
300,000,000 = wavelength (in metres) x frequency (in Hertz).

I'm sure you can do the maths .
Thanks, yes I vaguely remember something like this.. but is it really this easy?

299 792 458/(156.8*1000000)=1.912m

1.912/4=0.478m up and 0.478m down and 0.956m total length if my math is right.

How much would you lose by rounding up to 1 meter?

How would a antenna like this compare to a "proper" antenna?

What sort of gain is it?

Cheers

Ben
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Old 14-07-2014, 08:47   #20
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Re: VHF Radio - Tricks

Quote:
Originally Posted by Snowpetrel View Post
Anybody got the proper length to make your own vhf antenna. I saw instructions once in a vhf manual. Sounded like you peeled the coax away a set length to expose the core, and did some Thing with the cover? Cant quite remember what. But that would be a handy trick if it worked ok.
468/Freq in Mhz/2 for a 1/4 wave vertical in feet.

So, 156.8 Mhz is ch 16 VHF

468 / 156.8 = 2.98 / 2 = 1.49 ft or about 18"
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Old 14-07-2014, 11:43   #21
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Re: VHF Radio - Tricks

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Originally Posted by Canibul View Post
Good info. What's the informed thinking on antenna location on the mast? I'm a little gun shy, as the lightning bolt that took all our electronics out seems to have started with the VHF antenna on the top of the mast.
Our vhf antenna was vaporized by a lightning strike, and the vhf knocked out. But I had an 'emergency' suction cup antenna with 10 feet of cable and a connector for my handheld. Worked better than expected. It was made by Shakespeare, dont know if they still make it.

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Old 15-07-2014, 04:32   #22
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Re: VHF Radio - Tricks

Quote:
Originally Posted by Snowpetrel View Post
Thanks, yes I vaguely remember something like this.. but is it really this easy?

299 792 458/(156.8*1000000)=1.912m

1.912/4=0.478m up and 0.478m down and 0.956m total length if my math is right.

How much would you lose by rounding up to 1 meter?

How would a antenna like this compare to a "proper" antenna?


What sort of gain is it?

Cheers

Ben
It is a "proper antenna"

How much you lose depends on the "Q" factor of the antenna and right now I forget the Q of such a "simple" 1/2 wave dipole but it isn't super high - so you won't lose too much. If you like, do the maths to find the frequency of the 1 meter antenna and compare that to 156.8MHz.

But why round up; my tape measure is just as good at 956 mm as it is at 1,000 mm .

Gain is about 2 dB if I recall correctly.
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Old 15-07-2014, 06:10   #23
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Re: VHF Radio - Tricks

Quote:
Originally Posted by transmitterdan View Post
...For channel 16 (156.8MHz) the upper exposed center is about 18.75 inches long (to the tip of any lanyard loop) and the folded back outer conductor is about 15 inches.
Quote:
Originally Posted by Wotname View Post
It is a "proper antenna"

But why round up; my tape measure is just as good at 956 mm as it is at 1,000 mm .

Gain is about 2 dB if I recall correctly.
Many thanks for the good tips, so in metric for us poor Aussies, that makes it 478mm up and 382mm down with the coax taped in close to the wire should work. It could be hung from a halyard and gravity would hold it vertical. Very cool because I have had far to many antenna failures on deliveries and this is a simple compact emergency antenna to chuck in my kit bag rather than the short whip. Does it matter what impedance coax is used?

Cheers
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Old 15-07-2014, 06:23   #24
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Re: VHF Radio - Tricks

Quote:
Originally Posted by Snowpetrel View Post
Many thanks for the good tips, so in metric for us poor Aussies, that makes it 478mm up and 382mm down with the coax taped in close to the wire should work. It could be hung from a halyard and gravity would hold it vertical. Very cool because I have had far to many antenna failures on deliveries and this is a simple compact emergency antenna to chuck in my kit bag rather than the short whip. Does it matter what impedance coax is used?

Cheers
It is usual to use 50 ohm coax.
The real gurus might suggest matching baluns and so forth but it works just fine with the regular stuff.
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Old 15-07-2014, 15:06   #25
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Re: VHF Radio - Tricks

Yes, 50 ohm coax is what you want. There are formulas somewhere for 75 ohm coax which is common in satellite systems. I forget those formulas though.
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Old 16-07-2014, 03:39   #26
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Re: VHF Radio - Tricks

Quote:
Originally Posted by transmitterdan View Post
What Wotname is suggesting can work. A bit more detail might help visualize. What you want to end up with is an exposed section of the center conductor at the end. The original shield that was around the center is folded back down around the outside of the coax. The dimension given for the upper exposed center conductor is correct. However you will find that the VSWR is better if the exposed outer that is folded back is about 80% of the length of the exposed center. The reason is that the black jacket of the coax has a velocity factor of about 0.8. That just means the speed of light is slower by 0.8 inside the black plastic. The folded back shield should be as tight against the black cover as you can make it. Wrapping with black electrical tape can help. Make the center conductor a couple cm (1 inch) longer and make a loop. Tie a piece of small stuff to the loop and use this with a halyard to raise the homemade antenna. If you get the lengths right it will transmit/receive as well as most commercial antennas you can buy but it will not be as rugged. Even if you don't get the lengths just right it will work surprisingly well when hoisted near the top of the mast.

For channel 16 (156.8MHz) the upper exposed center is about 18.75 inches long (to the tip of any lanyard loop) and the folded back outer conductor is about 15 inches.
Thanks for the additional information. I hadn't considered the velocity factor before but it makes sense .

I will have to check the SWR next time I am messing around with this stuff
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Old 16-07-2014, 07:08   #27
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Re: VHF Radio - Tricks

<<< It could be hung from a halyard and gravity would hold it vertical. Very cool because I have had far to many antenna failures on deliveries and this is a simple compact emergency antenna to chuck in my kit bag rather than the short whip. >>>

I like it!

18.75" core, 15" coax.

That 15" braid is easily be sealed with a few pieces of overlapping Ancor heat shrink tubing.

The tip of the core, too can be sealed with a loop of good cord protruding from the tip (big enough for a shackle). The glue from a piece of heavy duty heat shrink tubing will hold it in place.

As long as there is something to hoist up, this will work. The metz can be taped or zip tied to the top of a whisker pole, oar, etc.

In the case of a sailboat hundreds of miles offshore whose electronics have all been knocked out by, say, water intrusion that wants maximum range..... would any multiple multiple of 18.75"/15" increase the jury-rigged antenna powered by a handheld?

Say 56.25" core and 45" braid folded down??
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Old 16-07-2014, 08:02   #28
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Re: VHF Radio - Tricks

Quote:
Originally Posted by Canibul View Post
Good info. What's the informed thinking on antenna location on the mast? I'm a little gun shy, as the lightning bolt that took all our electronics out seems to have started with the VHF antenna on the top of the mast. This boat came with one of those fuzzy lightning things up there, too. The metal VHF whip was sticking up higher than the fuzzy lightning thing. The lightning seems to have liked that, and I assume it used the braid of the coax to start it's very brief but thorough round of mischief as it dove into the sea through our boat.

I've been thinking of mounting a VHF antenna upside down, under the radar antenna mount. This would put it thirty feet above the water, but it would be shielded by the mast behind it. So, I wouldn't be able to talk to people behind me as well. This is inconvenient, but not a show stopper. Rarely do people behind me try to call. And I would expect a reduced range aft, not completly blanked. I could alter course long enough for a clear signal path if needed. Are these correct assumptions?

It's seeming that putting the VHF whip the highest thing on the mast is good for beam pattern, but does it attract lightning?

Anything put high on the mast is at risk. Another thread on this forum (you could search) described a vhf setup that could easily be raised and lowered, depending on circumstances.
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Old 16-07-2014, 16:49   #29
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Re: VHF Radio - Tricks

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Originally Posted by longbordz View Post
In the case of a sailboat hundreds of miles offshore whose electronics have all been knocked out by, say, water intrusion that wants maximum range..... would any multiple multiple of 18.75"/15" increase the jury-rigged antenna powered by a handheld?

Say 56.25" core and 45" braid folded down??

If hundreds of miles offshore you want a SSB radio. VHF will never be good for hundreds of miles no matter what antenna you have. The range is limited by the curvature of the earth. The HF bands do not have that limitation.
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Old 16-07-2014, 17:16   #30
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Re: VHF Radio - Tricks

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Originally Posted by longbordz View Post
...............
In the case of a sailboat hundreds of miles offshore whose electronics have all been knocked out by, say, water intrusion that wants maximum range..... would any multiple multiple of 18.75"/15" increase the jury-rigged antenna powered by a handheld?

Say 56.25" core and 45" braid folded down??
No, sorry
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