Marine Radio using Aviation Antennae

Ken Mattsson wrote in message ...
While on the subject of antennas, having built a couple of Jim´s plumber
delights, could this antenna design in principle be used for any bands or
can it only be built for center frequencies within the VHF-band?
Cheers, Ken
Finland


Every antenna I have seen can be built to any frequency. Do it by adjusting
the dimensions of the antenna inversely proportional to the frequency. for
example if you want the antenna to work at twice the frequency then the
dimensions should be half. Likewise if you want to use the antenna at half
the frequency then the dimensions should be doubled.

That is true...to a crude first approximation. The thickness of the antenna as
a function of frequency and antenna length is not so linear. The actual
calculation goes:

Calculate the THEORETICAL length as a submultiple of the wavelength.
Then do the finagle fatness factor for element thickness.

Jim


"Craig Davidson"
shared these priceless pearls of wisdom:

-
-Ken Mattsson wrote in message ...
-While on the subject of antennas, having built a couple of Jim´s plumber
-delights, could this antenna design in principle be used for any bands or
-can it only be built for center frequencies within the VHF-band?
-Cheers, Ken
-Finland
-
-
-Every antenna I have seen can be built to any frequency. Do it by adjusting
-the dimensions of the antenna inversely proportional to the frequency. for
-example if you want the antenna to work at twice the frequency then the
-dimensions should be half. Likewise if you want to use the antenna at half
-the frequency then the dimensions should be doubled.
-

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

find a ham radio operator in your area with a VHF capable SWR meter
(or just order a 2 meter swr meter from AES) if the SWR (standing
wave ratio) is greater then 2:1 then the antenna is not well enough
matched for even casual use. A good match is from say 1.1:1 to 1.40:1.
SWR meter hooks between the radio and the antenna and measures
transmitted and reflected power (well , sort of, the real explanation
would take a couple of pages). If the antenna isnt a good transmitter
at a particular frequency, then its a good reflector of RF back down
the cable. That RF has to go somewhere and that somewhere is back
into the final stage of the transmitter, causing heat and maybe
burnout.
Most modern marine HTs are designed with a circuit that backs off the
TX power or shuts down the transmitter if tyou have a bad match.

If this is for use at altitude in a non safety of life application, IE
calling your wife on the boat, you can get away with a crappy swr.
However keep in mind that if you transmit from any reasonable
altitude, your range is increased dramtically, and the marine RADIO is
FM, not AM, and the strongest singal wins on FM, unlike AM whjere they
overlap, so using a a decent marine radio at 5000 feet above San Fran,
you could end up capturing the channel down the whole west coast.
Probably somewhat illegal unless your doing some form of air to ground
work as part of your flying.

if your just gonna receive, the airband antenna will be just fine.

Steve Roberts , N8VKD

Horsefeathers. A 2:1 VSWR transmits 90% of the applied power. A 3:1 VSWR
transmits 75% of the applied power. Have you actually ever MEASURED a
commercial aircraft band antenna from bandedge to bandedge as installed on an
aircraft?

Jim


(Steve Roberts)
shared these priceless pearls of wisdom:

-find a ham radio operator in your area with a VHF capable SWR meter
-(or just order a 2 meter swr meter from AES) if the SWR (standing
-wave ratio) is greater then 2:1 then the antenna is not well enough
-matched for even casual use.
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

As I'm sure you know, and to be fair to the previous poster, you are
implying 90% of full power for 2:1 VSWR. As he stated any decent
transmitter has VSWR protection so it will back off the power to
protect itself. You will then only transmitt 90% of what the
transmitter is now putting out!

Do you know what transmitter output power would you realistically
expect in these conditions?

Personally I have accept 3:1 VSWR for occasional use as long as the PA
is protected. One time on holiday I lost one half of my 14MHz inverted
V dipole but still manage to reach UK from Southern Spain with 10W
SSB. Don't even ask what the VSWR meter was reading, it looked like it
was all being reflected!

On Mon, 11 Aug 2003 07:45:34 -0700, Jim Weir wrote:

Horsefeathers. A 2:1 VSWR transmits 90% of the applied power. A 3:1 VSWR
transmits 75% of the applied power. Have you actually ever MEASURED a
commercial aircraft band antenna from bandedge to bandedge as installed on an
aircraft?

Jim


(Steve Roberts)
shared these priceless pearls of wisdom:

-find a ham radio operator in your area with a VHF capable SWR meter
-(or just order a 2 meter swr meter from AES) if the SWR (standing
-wave ratio) is greater then 2:1 then the antenna is not well enough
-matched for even casual use.
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com



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