Question on transmitter/antenna mismatch compatibility
 

I would like to operate a 5 watt 151. MHz mobile transceiver into a
ball and spring low band whip antenna similar to the old bumper mount
CB antennas except that the antenna would be mounted directly to the
top of the rear fender of a large 1960's American car and needs to be
around 5-7 feet high. I need to use this type of antenna to give it
the appearance of an old police car.

Is it possible to do this effectively without damaging the
transmitter. I realize that this is an amateur radio board but I
thought that amateurs would be the best group to ask. Thank you.

Bruno

On 27 May 2004 19:06:26 -0700, (Bruno) wrote:

I would like to operate a 5 watt 151. MHz mobile transceiver into a
ball and spring low band whip antenna similar to the old bumper mount
CB antennas except that the antenna would be mounted directly to the
top of the rear fender of a large 1960's American car and needs to be
around 5-7 feet high. I need to use this type of antenna to give it
the appearance of an old police car.

Is it possible to do this effectively without damaging the
transmitter. I realize that this is an amateur radio board but I
thought that amateurs would be the best group to ask. Thank you.

Bruno

Hi Bruno,

MURS allows only 2W effective radiated power, not 5W going to an
antenna. Further, this is into a simple antenna. Yeah, I know,
yours seems to be a simple one, however as long as it is (5-7 feet
high), it is both a poor match (almost no power out), and would have
gain (a power multiplying factor) if you did match it (making it more
powerful, hence the term "effective" radiated power). Even more
unfortunate is that this gain (and most sensitivity for receiving)
would be up into the sky! This is like drawing three cards to an
inside straight: antenna too long, poor match, deaf too.

Now, this sounds like a lot of contradictions I am sure. The simplest
antenna would be roughly 18 inches tall. Use the AM radio antenna for
that, use a legal transmitter, and put up your buggy whip to impress
the buddies. As an alternative, the antenna length you are describing
is quite close to those used in the CB bands which would work just as
well, if not better (as long as who you talk to use the same bands).

By the way, are you located near Norwalk Connecticut?

73's
Richard Clark, KB7QHC

Hi Richard,

Thanks for the reply. Sorry, no, I'm not in Connecticut.

I believe the radio that I have is "rated" at 5 watts but
automatically cuts back to 2 watts when used on any of the "dot" MURS
frequencies for which no license is required as long as there is no
interference to licensed stations.

For appearance reasons the ONLY antenna that I can have on the car is
the spring/ball whip. There won't even be an AM band antenna on the
car. CB radios are out for a number of reasons. For one the 108" CB
whip is just as incorrect as the 18" whip. Plus I already have the
MURS radios- a mobile and handheld and my need is only to coordinate
legitimate communications with other existing MURS units at events. I
have a handheld that I can use but a mobile unit would just be easier
and safer to use while moving.

I know that antennas can be "electrically lengthened" with coils, is
there no way to "electrically shorten" them perhaps with some sort of
matching transformer?

Bruno


Hi Bruno,

MURS allows only 2W effective radiated power, not 5W going to an
antenna. Further, this is into a simple antenna. Yeah, I know,
yours seems to be a simple one, however as long as it is (5-7 feet
high), it is both a poor match (almost no power out), and would have
gain (a power multiplying factor) if you did match it (making it more
powerful, hence the term "effective" radiated power). Even more
unfortunate is that this gain (and most sensitivity for receiving)
would be up into the sky! This is like drawing three cards to an
inside straight: antenna too long, poor match, deaf too.

Now, this sounds like a lot of contradictions I am sure. The simplest
antenna would be roughly 18 inches tall. Use the AM radio antenna for
that, use a legal transmitter, and put up your buggy whip to impress
the buddies. As an alternative, the antenna length you are describing
is quite close to those used in the CB bands which would work just as
well, if not better (as long as who you talk to use the same bands).

By the way, are you located near Norwalk Connecticut?

73's
Richard Clark, KB7QHC

Bruno,
To keep appearances 'correct', how about using a
'hidden' antenna of some sort? You can still have a
'dummy' antenna on the rear fender.
How you 'hide' the antenna depends on what you have
to work with and your ingenuity/imagination, and how
much 'compromise' in appearance you're willing to put
up with. If you think a fibreglass whip instead of a
steel one would look okay, it would be fairly simple.
Just snip the internal wire to the right length. Well,
maybe not so 'simple', but still possible.
Or, how about using very small conductors and glueing
the antenna to a window (burgler alarm tape?)?
'Doc

PS - Put a 2 meter antenna on a '51 Ford p/u like that,
it worked 'okay'...

You could cut the whip to 3/4 wavelength, ca 1.5 meters which could give
reasonable swr.

73 de Hans SM3PXG
Sweden



Bruno wrote:
Hi Richard,

Thanks for the reply. Sorry, no, I'm not in Connecticut.

I believe the radio that I have is "rated" at 5 watts but
automatically cuts back to 2 watts when used on any of the "dot" MURS
frequencies for which no license is required as long as there is no
interference to licensed stations.

For appearance reasons the ONLY antenna that I can have on the car is
the spring/ball whip. There won't even be an AM band antenna on the
car. CB radios are out for a number of reasons. For one the 108" CB
whip is just as incorrect as the 18" whip. Plus I already have the
MURS radios- a mobile and handheld and my need is only to coordinate
legitimate communications with other existing MURS units at events. I
have a handheld that I can use but a mobile unit would just be easier
and safer to use while moving.

I know that antennas can be "electrically lengthened" with coils, is
there no way to "electrically shorten" them perhaps with some sort of
matching transformer?

Bruno


Hi Bruno,

MURS allows only 2W effective radiated power, not 5W going to an
antenna. Further, this is into a simple antenna. Yeah, I know,
yours seems to be a simple one, however as long as it is (5-7 feet
high), it is both a poor match (almost no power out), and would have
gain (a power multiplying factor) if you did match it (making it more
powerful, hence the term "effective" radiated power). Even more
unfortunate is that this gain (and most sensitivity for receiving)
would be up into the sky! This is like drawing three cards to an
inside straight: antenna too long, poor match, deaf too.

Now, this sounds like a lot of contradictions I am sure. The simplest
antenna would be roughly 18 inches tall. Use the AM radio antenna for
that, use a legal transmitter, and put up your buggy whip to impress
the buddies. As an alternative, the antenna length you are describing
is quite close to those used in the CB bands which would work just as
well, if not better (as long as who you talk to use the same bands).

By the way, are you located near Norwalk Connecticut?

73's
Richard Clark, KB7QHC

On 27 May 2004 19:06:26 -0700, (Bruno) wrote:

I would like to operate a 5 watt 151. MHz mobile transceiver into a
ball and spring low band whip antenna similar to the old bumper mount
CB antennas except that the antenna would be mounted directly to the
top of the rear fender of a large 1960's American car and needs to be
around 5-7 feet high. I need to use this type of antenna to give it
the appearance of an old police car.

Is it possible to do this effectively without damaging the
transmitter. I realize that this is an amateur radio board but I
thought that amateurs would be the best group to ask. Thank you.

Bruno


-------------------------------------------------------------
Just a shot in the dark, Can ya stuff a rubber ducky in the Bubble Gum
dome? Or maybe a 74" + or- whip with GROUNDED gutter clip.
or a period correct GE Highband whip in addition to the ball and spring.
73
Greg
WG8Z

Greg Z
to thine own sound be true

Bruno wrote:
I know that antennas can be "electrically lengthened" with coils, is
there no way to "electrically shorten" them perhaps with some sort of
matching transformer?

Unfortunately, a too-long vertical tends to send the radiation
up at a high angle. EZNEC sez a 6' vertical used on 151 MHz
will have a take-off-angle of about 38 degrees - not good for
vhf mobile.

RS makes a fiberglass CB whip. How about cutting one of those
to your desired length and drilling through the center conductor
at the proper 1/4WL point? Epoxy the hole? Paint the fiberglass
to make it look like metal?
--
73, Cecil http://www.qsl.net/w5dxp



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On Fri, 28 May 2004 08:20:11 -0500, Cecil Moore
wrote:

Bruno wrote:
I know that antennas can be "electrically lengthened" with coils, is
there no way to "electrically shorten" them perhaps with some sort of
matching transformer?

Unfortunately, a too-long vertical tends to send the radiation
up at a high angle. EZNEC sez a 6' vertical used on 151 MHz
will have a take-off-angle of about 38 degrees - not good for
vhf mobile.

RS makes a fiberglass CB whip. How about cutting one of those
to your desired length and drilling through the center conductor
at the proper 1/4WL point? Epoxy the hole? Paint the fiberglass
to make it look like metal?

Hi Bruno,

This is your best solution both for a correct length antenna and a
cosmetic appearance.

73's
Richard Clark, KB7QHC

In message , Richard Clark
writes
On 27 May 2004 19:06:26 -0700, (Bruno) wrote:

I would like to operate a 5 watt 151. MHz mobile transceiver into a
ball and spring low band whip antenna similar to the old bumper mount
CB antennas except that the antenna would be mounted directly to the
top of the rear fender of a large 1960's American car and needs to be
around 5-7 feet high. I need to use this type of antenna to give it
the appearance of an old police car.

Is it possible to do this effectively without damaging the
transmitter. I realize that this is an amateur radio board but I
thought that amateurs would be the best group to ask. Thank you.

Bruno

Hi Bruno,

MURS allows only 2W effective radiated power, not 5W going to an
antenna. Further, this is into a simple antenna. Yeah, I know,
yours seems to be a simple one, however as long as it is (5-7 feet
high), it is both a poor match (almost no power out), and would have
gain (a power multiplying factor) if you did match it (making it more
powerful, hence the term "effective" radiated power). Even more
unfortunate is that this gain (and most sensitivity for receiving)
would be up into the sky! This is like drawing three cards to an
inside straight: antenna too long, poor match, deaf too.

Now, this sounds like a lot of contradictions I am sure. The simplest
antenna would be roughly 18 inches tall. Use the AM radio antenna for
that, use a legal transmitter, and put up your buggy whip to impress
the buddies. As an alternative, the antenna length you are describing
is quite close to those used in the CB bands which would work just as
well, if not better (as long as who you talk to use the same bands).

By the way, are you located near Norwalk Connecticut?

73's
Richard Clark, KB7QHC


Actually, there is no problem.

As it stands, at the impedance looking into the coax (TX end) won't be
50 ohms. To keep the TX happy, all you need to do is to insert a simple
matching circuit (at the TX end) to transform whatever the impedance is
to 50 ohms.

This won't change the impedance looking into the coax, and the SWR on
the coax will be the same. As a result, the loss on the coax will be
higher than if the SWR was 1:1. However, as the length of the coax won't
be that great (even in those large American cars!), I doubt if you'll
notice it.

For a suitable matcher, I suggest you get one of those very cheap small
CB type (the kind in with the two compression trimmers, often found in a
junk box), and rebuild it into a C-L-C pi configuration (if it isn't
already). The coil will probably be about 4 turns 1/4" dia (some
experimentation will be necessary). To adjust the matcher, insert an SWR
meter between the TX and the matcher, and connect the matcher to the
coax.

Cheers,
Ian.
--

Before GOOGLE Groups does funny things again I'd like to thank you all
for your input and help. The trick about the fiberglass antenna sounds
interesting. There are a lot of ideas to experiment with here, I'll
have to see which one works best.

Thanks again,

Bruno