Can one use an insulated backstay as a VHF antenna? If so has anyone rigged it up? Or does only single sideband work?
carrd@erols.com
VHF on backstay
Moderator: Jim Walsh
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Lyn Heiges
Re: VHF on backstay
The portion of the RF (radio frequency) spectrum which is used for the marine VHF is near the "2-meter" wavelength which means that the length of one complete wavelength is about 38-inches and the typical vertical antenna used is 1/2 the wavelength or about 19-inches when used in 1/2 wavelength antennae. 5/8 wavelength antennae are often used on powerboats where the antenna is relatively vertical at all times. These may be colinear configurations which are often placed in helical structural radiators of approximately 72 to 96-inches in length.Don Carr wrote: Can one use an insulated backstay as a VHF antenna? If so has anyone rigged it up? Or does only single sideband work?
The use of the backstay for either low-band MF (medium frequency) or HF (high frequency) applications usually associated with SSB (single-sideband) are compromises at best. This is because the polarization of the broadcast signal is orthoganal to the radiator (the wire or whip) so the signal from a backstay is neither vertically or horizontally polarized. The benefit of use of the backstay is the length which is usually excellent in comparison to other options for the transmission device required for the portion of the spectrum which may be used where the signal may require 20 to 60 foot optimal antenna lengths.
As to can you use the backstay. Sure! You will need an antenna tuner and have insulators installed in the backstay. Is it worth it for VHF? Absolutely NOT! You don't get the height which desirable for VHF transmission as it is virtually "line-of-sight" and the signal losses you are likely to suffer are much greater than with good coaxial cable of the RG-214 type or better low-loss RG-8U.
Good Luck!
Lyn Heiges, WA3RRK
CD28 MOON CHILD
CD27 GUILLEMOT
lheiges@compuserve.com
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John Nuttall
Re: VHF on backstay
Hi Don,
I'll try to answer that. Although I was a Radioman in the Naval Reserve (early 70's), I have forgotten much I was tought ( It aint like riding a bike) but here goes:
VHF frequencies are pretty much "line-of-sight" and limited distance. In other words the transmitted signal leaves the source antenna and travels in a rather straight line. Because of the curvature of the earth this limits the range over open water to about 15 miles, give or take depending on the strength of the signal, height of the antennas (sending and recieving)and a few other factors.The signals actually continue on that straight line out into the atmosphere, but VHF signals normally do not "skip" off the atmosphere and bounce back down to earth, like SSB.
All antennas need to be sized proportionally to the wave of the signal generated at the particular frequency you are transmitting/recieving. VHF signals are rather short and require shorter antennas than do say HF and SSB signals. The antennas are sized ( 1/4 wave, 1/2 wave etc ) to the signal wave size, and tuned electronically to match the wave size. A backstay is too long for VHF signal. (I suppose a backstay would be better than no antenna in an emergency). Mast head VHF antennas will give you the best results.
You could turn a section of your upper backstay into a VHF antenna by isolating the correct length of backstay, near the mast head with insulators, but it would be easier and cheaper and stronger (rig wise ) to just but a VHF antenna and install it at the top of the mast.
You could install a signal splitter too and the VHF antenna would serve double duty for your FM tunes, and even get mediocre (sp?) TV reception.
I hope this helps. It's a crude explaination, but correct. Maybe there are some radio buffs who could help.
John Nuttall
s/v "Aimless"
CD31 #28
Oriental, NC
nuttallj@msn.com
I'll try to answer that. Although I was a Radioman in the Naval Reserve (early 70's), I have forgotten much I was tought ( It aint like riding a bike) but here goes:
VHF frequencies are pretty much "line-of-sight" and limited distance. In other words the transmitted signal leaves the source antenna and travels in a rather straight line. Because of the curvature of the earth this limits the range over open water to about 15 miles, give or take depending on the strength of the signal, height of the antennas (sending and recieving)and a few other factors.The signals actually continue on that straight line out into the atmosphere, but VHF signals normally do not "skip" off the atmosphere and bounce back down to earth, like SSB.
All antennas need to be sized proportionally to the wave of the signal generated at the particular frequency you are transmitting/recieving. VHF signals are rather short and require shorter antennas than do say HF and SSB signals. The antennas are sized ( 1/4 wave, 1/2 wave etc ) to the signal wave size, and tuned electronically to match the wave size. A backstay is too long for VHF signal. (I suppose a backstay would be better than no antenna in an emergency). Mast head VHF antennas will give you the best results.
You could turn a section of your upper backstay into a VHF antenna by isolating the correct length of backstay, near the mast head with insulators, but it would be easier and cheaper and stronger (rig wise ) to just but a VHF antenna and install it at the top of the mast.
You could install a signal splitter too and the VHF antenna would serve double duty for your FM tunes, and even get mediocre (sp?) TV reception.
I hope this helps. It's a crude explaination, but correct. Maybe there are some radio buffs who could help.
John Nuttall
s/v "Aimless"
CD31 #28
Oriental, NC
nuttallj@msn.com
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John Nuttall
Re: VHF on backstay
Wow, that was wierd! Lyn's reply posted at the same time as mine.
Anyway Don, Lyn has all the technical stuff down! And his point about orthagonal axis is one I had forgotten - the significance? - the antenna needs to be verticaly for optimum performance.
Now that you know you need to install a masthead antenna, do yourself a favor and install masthead lighting while you are at it. Or at least "pre-wire" for lighting, wind vane , wind speed instrumentation, etc ( whatever you might want in the future) at the same time you run the antenna leads. It will make for fewer hassles in the future!
Bye (again),
John
nuttallj@msn.com
Anyway Don, Lyn has all the technical stuff down! And his point about orthagonal axis is one I had forgotten - the significance? - the antenna needs to be verticaly for optimum performance.
Now that you know you need to install a masthead antenna, do yourself a favor and install masthead lighting while you are at it. Or at least "pre-wire" for lighting, wind vane , wind speed instrumentation, etc ( whatever you might want in the future) at the same time you run the antenna leads. It will make for fewer hassles in the future!
Bye (again),
John
nuttallj@msn.com
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Don Carr
Re: VHF on backstay..Thanks for info.
Thanks for the information. As usual this board has proven to be a valuable resource. Will be untepping my mast for other work after the holiday weekend.Don Carr wrote: Can one use an insulated backstay as a VHF antenna? If so has anyone rigged it up? Or does only single sideband work?
carrd@erols.com