John Smith wrote:
A half-wave fed with a gamma would need no groundplane... you could build
one better than any you could purchase...

Warmest regards,
John

"Ed" wrote in message
. 92.175...

I haven't fired up EZNEC yet, but I was wondering how a standard 3dBi
marine whip would perform on the 2m ham bands. Since the marine
frequencies are 156.050-157.425 MHz, what could I expect at 144-148

MHz?

If it won't cut it, then does anyone have any recommendations for a
dual-band antenna (commercial or home-built)?

Jon


Jon,

I would expect the marine antenna to perform poorly, due to high SWR.
However, I am not sure of your second question. Are you looking for a
2M/440 antenna for a boat, or for automobile mount? A number of vendors
(Larson, Antenex, Comet?, etc.) sell dual band mobile antennas for those
two ham bands. For a boat, I don't know of any dualband stuff, but those
same vendors sell a base loaded half wave antenna for single ham bands
that doesn't require any ground plane. Good Luck.


Ed K7AAT




John/Ed,

It's for a sailboat (all this has been hashed over in
rec.boats.electronics - I mistakenly cross-posted to rec.radio.antenna,
rather than rec.radio.amateur.antenna, so you guys missed the thread
there). The general consensus is it would be easier to use two antennas,
but I'd have to enlarge the holes in the compression post under the mast
and the mast itself in order to run another cable. If I could find a
pair of diplexers, then at least both radio/antenna combos could share
the same feedline up the mast...

Anyone know of a 2m/70cm diplexer with tight enough filter slope to put
the marine band on the same side as the 70cm, or one specific to 2m/marine?

Jon KB1HTW

It's for a sailboat (all this has been hashed over in
rec.boats.electronics - I mistakenly cross-posted to
rec.radio.antenna, rather than rec.radio.amateur.antenna, so you guys
missed the thread there). The general consensus is it would be easier
to use two antennas, but I'd have to enlarge the holes in the
compression post under the mast and the mast itself in order to run
another cable. If I could find a pair of diplexers, then at least both
radio/antenna combos could share the same feedline up the mast...

Anyone know of a 2m/70cm diplexer with tight enough filter slope to
put the marine band on the same side as the 70cm, or one specific to
2m/marine?

Jon KB1HTW



John, I know that some duplexer manufacturer such as Tx/Rx (
http://www.txrx.com ) make a pretty small repeater duplexer for VHF that
would handle the split between amateur and marine band.... if you were to
separate the cavities and use them in your manner... but not sure if they
would still be frequency broad enough, or small enough if space is really
cramped in the cabin. Something to research, anyway.

QUESTION, though. What size or type of cable is currently run up in
the mast?

Ed

Ed wrote:


It's for a sailboat (all this has been hashed over in
rec.boats.electronics - I mistakenly cross-posted to
rec.radio.antenna, rather than rec.radio.amateur.antenna, so you guys
missed the thread there). The general consensus is it would be easier
to use two antennas, but I'd have to enlarge the holes in the
compression post under the mast and the mast itself in order to run
another cable. If I could find a pair of diplexers, then at least both
radio/antenna combos could share the same feedline up the mast...

Anyone know of a 2m/70cm diplexer with tight enough filter slope to
put the marine band on the same side as the 70cm, or one specific to
2m/marine?

Jon KB1HTW




John, I know that some duplexer manufacturer such as Tx/Rx (
http://www.txrx.com ) make a pretty small repeater duplexer for VHF that
would handle the split between amateur and marine band.... if you were to
separate the cavities and use them in your manner... but not sure if they
would still be frequency broad enough, or small enough if space is really
cramped in the cabin. Something to research, anyway.

QUESTION, though. What size or type of cable is currently run up in
the mast?

Ed


Ed,

It's marine-grade RG-8x. The antenna is at the top of the mast, 36 ft
above the cabin top.

It's marine-grade RG-8x. The antenna is at the top of the mast, 36 ft
above the cabin top.



My thought, in asking this questions, was replacing the current coax
with two of smaller diameter, since you said the current hole in the mast
was too small to add one. Commerically, we often use a RG-58 type coax
that has a white Teflon jacket, commonly referred to as Motorola White
Teflon coax. It is considerably smaller than RG-8X and I would hazzard to
say you might be able to pull a pair of them up, using the original RG-8X
as a pull line. You will need to obtain connectors with the proper size
ferrule for it as it is smaller than RG-58. Good Luck.

Ed K7AAT

Ed wrote:


It's marine-grade RG-8x. The antenna is at the top of the mast, 36 ft
above the cabin top.




My thought, in asking this questions, was replacing the current coax
with two of smaller diameter, since you said the current hole in the mast
was too small to add one. Commerically, we often use a RG-58 type coax
that has a white Teflon jacket, commonly referred to as Motorola White
Teflon coax. It is considerably smaller than RG-8X and I would hazzard to
say you might be able to pull a pair of them up, using the original RG-8X
as a pull line. You will need to obtain connectors with the proper size
ferrule for it as it is smaller than RG-58. Good Luck.

Ed K7AAT


Ed,

Had a major case of memory corruption - my coax is RG-58, not the larger
8X ;-). I think I'll admit defeat and just find a way to shoehorn
another coax up the mast. Maybe I'll just drill thru the cabintop and
epoxy a stainless threaded SO-239 nipple thru it so the balsa core of
the fiberglass won't suck up moisture like a sponge. Then just make
another run up the mast.

The problem with drilling a larger hole in the mast step is that it's
harder to seal against moisture when you have lots of cables in a
relatively large hole. Water inevitably gets in...

Had a major case of memory corruption - my coax is RG-58, not the
larger 8X ;-). I think I'll admit defeat and just find a way to
shoehorn another coax up the mast. Maybe I'll just drill thru the
cabintop and epoxy a stainless threaded SO-239 nipple thru it so the
balsa core of the fiberglass won't suck up moisture like a sponge.
Then just make another run up the mast.

The problem with drilling a larger hole in the mast step is that it's
harder to seal against moisture when you have lots of cables in a
relatively large hole. Water inevitably gets in...



Let me reiterate, the Motorola white Teflon jacketed RG-58 type coax is
slightly smaller in diameter than regular RG-58 coax, but not any more
loss. Plus, the Teflon jacket is very slick, making pulling it in tight
quarters easier. I'd still suggest you use the original coax as a pull
line and pull up two of these Teflon jacketed coaxes.


Ed

Ed wrote:
Let me reiterate, the Motorola white Teflon jacketed RG-58 type coax is
slightly smaller in diameter than regular RG-58 coax, but not any more
loss. Plus, the Teflon jacket is very slick, making pulling it in tight
quarters easier. I'd still suggest you use the original coax as a pull
line and pull up two of these Teflon jacketed coaxes.

Teflon RG-303 is 0.170" in diameter.
--
73, Cecil http://www.qsl.net/w5dxp


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