Coax antenna for aircraft, help.
 

I saw the following antenna being made by some ultralight guys in San Diego.
I was wondering what to think of this. I am new to antennas and just
starting down the path to hamdom. Anything I need to know before starting
to experiment with making one? I don't want to fry my only radio, a Vertex
Standard VX-700.

Below are the plans for an antenna which does not require a ground plane.

This antenna is a 1/2 wave vertical dipole made out of RG-58/U coax and a
Radio shack FM radio antenna. The beauty of this design is that it is cheap,
simple, and easy to make with readily available materials. The radiator
should be 3', 9 1/2 inches long for 123.450 operation, but only the top foot
or two needs to be the telescoping FM radio antenna. Or, you can make the
entire radiator length out of regular antenna wire if you wish. The only
critical thing is the total length.

Start out with enough coax so that you will have enough to run up to the
radio from your antenna mounting location.

First, solder the telescoping antenna to the middle conductor on the coax.
Make sure to strip back the shield far enough so that it does not short to
the "radiator" or telescoping part of the antenna. Remember, you can make
the "radiator" entirely out of the center conductor on the coax, so if you
are going to do that, you must make sure the dimensions are correct. Next,
on the other end of the coax, I cut off a piece of coax that is going to be
the shorting stub. In all cases, I always make the lengths to the long side.
This way the antenna can be trimmed in to the desired frequency with a SWR
bridge.

To connect the shorting stub to the feedline, carefully remove a small
section of insulation from the center conductor (no more than 1/4 of an
inch). Solder and tape the inner conductor, then join and solder the
shields. To insure proper shielding, split a spare piece of braid lengthwise
on top of the shield connection, and carefully solder the shield together.
(Rather than disecting the coax cable and soldering, we used a BNC
connector for the shorting stub. We then trimed back for tuning and ended
up with almost no wire needed on the shorting stub. VERY SHORT.)

Don't forget to put a BNC connector on the radio end.

Finally, hang up your antenna in an open space, and tune it in with a SWR
meter. I have found that adjusting the length of the shorting stub has the
most affect in adjusting for minimum SWR but be careful. If you cut too much
off you will have to re-do it.

Radiator Length: 3' 9.5"
Radiator to shorting stub: 12.9"
Shorting Stub: 1.9" (imagine no BNC connector when
measuring)

QUESTIONS:

(1) So how do I calculate lengths for other frequencies? ie 121.5MHz,
146.73MHZ, 123.0 MHz
(2) Any chance of damaging my Vertex Standard VX-700 if I start playing
around with this?
(3) Where can I finde a descent but cheap SWR meter?


Thanks,
Carl

On Sat, 30 Oct 2004 22:10:10 -0700, "Carl J. Hixon"
wrote:
Hi Carl,

By the numbers:
(1) So how do I calculate lengths for other frequencies? ie 121.5MHz,
146.73MHZ, 123.0 MHz
By the proportion to the original frequency (this is called scalable).
This usually implies thickness too, but the variation of your
frequencies is slight enough to disregard. Besides, you need to
anticipate some trimming using the SWR meter you are asking for in
(3).

(2) Any chance of damaging my Vertex Standard VX-700 if I start playing
around with this?
Not particularly for low power and modest SWR (and probably not even
immodest SWR).

(3) Where can I finde a descent but cheap SWR meter?
Radio Shack.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Sat, 30 Oct 2004 22:10:10 -0700, "Carl J. Hixon"
wrote:
Hi Carl-

Tho, the rest are ok- must disagree on the SWR bridge, as some of those cheap
ones (Swan Electronics was a good example, among others) used lousey sampleing
circuits in their directional couplers. What does this do?? well, as these
are NOT even close to 50 ohm , and a substancial part of a 1/4 wave, actually
introduce a mismatch! It is easy to test for- simply terminate the antenna
with a 47 ohm 2 watt resistor, and measure the swr -should be 1:1-- then reverse
the meter and measure the swr -backwards (set the pot , in the REFLECTED position
then measure the swr in the Calibrate (or forward) position again should read 1:1
I have seen some (like the swan) reading greater than 3:1 at even 6 meters! Also,
on very high swrs, have actually seen "more" power return than was in the calibrate
position (a impossible situation !) This is caused you the lousey directional couplers
in the meter, itself!
These were made for CB, not for VHF! If it doesn't pass this test- would be a good idea
to try another brand or (perferaably) borrow a Bird (or equivalent). As information--
Jim NN7K

(3) Where can I finde a descent but cheap SWR meter?

Radio Shack.

73's
Richard Clark, KB7QHC


--
To reply, remove the NOSPAM

By the numbers:
(1) So how do I calculate lengths for other frequencies? ie 121.5MHz,
146.73MHZ, 123.0 MHz
By the proportion to the original frequency (this is called scalable).
This usually implies thickness too,

Thank you for the reply. Care to show me an example? I don't know which
lenghts are important, do I scale the radiator lenght, distance between
radiator and shorting stub, and / or shorting stub?

(3) Where can I finde a descent but cheap SWR meter?
Radio Shack.

I had checked at my local radio shack and the kid had no idea what I was
talking about. Walked around and said, "we must not have them." It's not a
very good store...only good at selling their toys, phones, etc. The Shack
isn't what it used to be. Can anybody recommend a brand / model for under
$100?

Thanks,
Carl

Carl wrote:

I had checked at my local radio shack and the kid had no idea what I was
talking about. Walked around and said, "we must not have them." It's not a
very good store...only good at selling their toys, phones, etc. The Shack
isn't what it used to be. Can anybody recommend a brand / model for under
$100?
Carl -

Try HRO - Diamond makes some nice meters for under $100.00 - for instance:

Item : SX-40C
Description : 15/150W 144-470 MHZ COMPACT WMTR
YOUR HRO PRICE $79.95

Lee Hopper, NB7F
nr Portland, OR

On Sun, 31 Oct 2004 17:50:28 -0800, "Carl J. Hixon"
wrote:
I had checked at my local radio shack and the kid had no idea what I was
talking about. Walked around and said, "we must not have them." It's not a
very good store...only good at selling their toys, phones, etc. The Shack
isn't what it used to be. Can anybody recommend a brand / model for under
$100?

Hi Carl,

A quick check at their web site reveals:
http://www.radioshack.com/iphraseget...Aoid&text=&sf=
or simply put SWR Meter into their search engine.

Lee's suggestion is just as good too.

As for simple instructions, the web abounds with them for 2M, you
simply scale any one of them to your frequency. You could build one
for less than $5 and it would mostly be for the SO-239 connector that
is used as the basis for a quarter wave vertical with four radials.

Use ordinary #14 solid wi
Vertical wire, 21"
each of 4 radials, 13"
droop the radials at 45°

....or so my quick measurements show. Your mileage may vary. Consult
other authorities for a sanity check.

73's
Richard Clark, KB7QHC

On Sun, 31 Oct 2004 17:50:28 -0800, "Carl J. Hixon"
wrote:


By the numbers:
(1) So how do I calculate lengths for other frequencies? ie 121.5MHz,
146.73MHZ, 123.0 MHz
By the proportion to the original frequency (this is called scalable).
This usually implies thickness too,

Thank you for the reply. Care to show me an example? I don't know which
lenghts are important, do I scale the radiator lenght, distance between
radiator and shorting stub, and / or shorting stub?

(3) Where can I finde a descent but cheap SWR meter?
Radio Shack.

I had checked at my local radio shack and the kid had no idea what I was
talking about. Walked around and said, "we must not have them." It's not a
very good store...only good at selling their toys, phones, etc. The Shack
isn't what it used to be. Can anybody recommend a brand / model for under
$100?

Thanks,
Carl

Carl,
I know that this isn't the answer you are seeking but I'll give it a
go anyway. Whay not just use a quarter wave? If your ultralight
aircraft has a metal framework and/or skin you could use that as the
'missing half' and just use a 1/4 wave whip. This is done in land
mobile all the time, a 1/4 wave whip that uses the vehicle body as the
ground plane. While I might be missing something by not knowing much
of anything about the aircraft you wish to mount this on it seems like
the simplest solution. I've built 3 from published measurements and
the SWR was acceptable on all (under 1.5:1) without pruning. Figure
the center frequency of where you plan to operate (on each band) cut
for that and call it a day. If you do wish to get the SWR meter
anyway another poster recommeded a Daiwa - I have one of their SWR
meters and concurr with his recommendation.
Howard

"Carl J. Hixon" wrote in message news:jo_gd.85932$hj.69553@fed1read07...
I saw the following antenna being made by some ultralight guys in San Diego.
I was wondering what to think of this. I am new to antennas and just
starting down the path to hamdom. Anything I need to know before starting
to experiment with making one? I don't want to fry my only radio, a Vertex
Standard VX-700.

Below are the plans for an antenna which does not require a ground plane.

This antenna is a 1/2 wave vertical dipole made out of RG-58/U coax and a
Radio shack FM radio antenna. The beauty of this design is that it is cheap,
simple, and easy to make with readily available materials. The radiator
should be 3', 9 1/2 inches long for 123.450 operation, but only the top foot
or two needs to be the telescoping FM radio antenna. Or, you can make the
entire radiator length out of regular antenna wire if you wish. The only
critical thing is the total length.

Start out with enough coax so that you will have enough to run up to the
radio from your antenna mounting location.

First, solder the telescoping antenna to the middle conductor on the coax.
Make sure to strip back the shield far enough so that it does not short to
the "radiator" or telescoping part of the antenna. Remember, you can make
the "radiator" entirely out of the center conductor on the coax, so if you
are going to do that, you must make sure the dimensions are correct. Next,
on the other end of the coax, I cut off a piece of coax that is going to be
the shorting stub. In all cases, I always make the lengths to the long side.
This way the antenna can be trimmed in to the desired frequency with a SWR
bridge.

To connect the shorting stub to the feedline, carefully remove a small
section of insulation from the center conductor (no more than 1/4 of an
inch). Solder and tape the inner conductor, then join and solder the
shields. To insure proper shielding, split a spare piece of braid lengthwise
on top of the shield connection, and carefully solder the shield together.
(Rather than disecting the coax cable and soldering, we used a BNC
connector for the shorting stub. We then trimed back for tuning and ended
up with almost no wire needed on the shorting stub. VERY SHORT.)

Don't forget to put a BNC connector on the radio end.

Finally, hang up your antenna in an open space, and tune it in with a SWR
meter. I have found that adjusting the length of the shorting stub has the
most affect in adjusting for minimum SWR but be careful. If you cut too much
off you will have to re-do it.

Radiator Length: 3' 9.5"
Radiator to shorting stub: 12.9"
Shorting Stub: 1.9" (imagine no BNC connector when
measuring)

QUESTIONS:

(1) So how do I calculate lengths for other frequencies? ie 121.5MHz,
146.73MHZ, 123.0 MHz

For a half wave it's 5616 ÷ frequency in Mhz = Radiator length in
inches.

For a quarter wave it's 2808 ÷ freq.

(2) Any chance of damaging my Vertex Standard VX-700 if I start playing
around with this?

No. It protects itself, don't worry about it.

(3) Where can I finde a descent but cheap SWR meter?

There is great beauty in the KISS theory. Don't bother with an swr
bridge for no-brainers like this with an HT, it's overkill for any
purposes, close enough is close enough. Cobble together two antennas,
one for the air band and one for the 2M ham band, swap antennas
depending on the band in use and be done with it which is easy with
BNC connectors.

The simplest cheapest type of VHF antenna is the ¼ wave coaxial "no
radials vertical" and is made from RG-58. Calculate the length of the
¼ wave radiator minus one inch to roughly correct for the velocity
factor and strip off the plastic jacket to that length plus ½ inch.
Don't strip the insulation off the center conductor. Then horse the
braided shield back down over the jacket of the coax toward the radio
end. Lay a length of nylon cord alongside the radiator which extends
some number of feet beyond the upper end of the radiator. Seal the
whole assembly with heat shrink tubing, plug the top end of it with
some Goop and hang it up somewhere handy. This is not a particularly
portable "shack on the belt" sort of solution but it's essentially
bulletproof.

I strung one of these up in the airframe of my Quicksilver MX in my
ultralight flying days and it didn't just work, it drove me batty, at
altitude I heard every weekender UNICOM chatterbox for a couple
hundred miles around and I quickly retreated back to the rubber duck
to get some "attenuation".

Thanks,
Carl

w3rv

"Carl J. Hixon" wrote in message
news:jo_gd.85932$hj.69553@fed1read07...
I saw the following antenna being made by some ultralight guys in San
Diego.
I was wondering what to think of this. I am new to antennas and just
starting down the path to hamdom. Anything I need to know before starting
to experiment with making one? I don't want to fry my only radio, a
Vertex
Standard VX-700.

Below are the plans for an antenna which does not require a ground plane.

This antenna is a 1/2 wave vertical dipole made out of RG-58/U coax and a
Radio shack FM radio antenna. The beauty of this design is that it is
cheap,
simple, and easy to make with readily available materials. The radiator
should be 3', 9 1/2 inches long for 123.450 operation, but only the top
foot
or two needs to be the telescoping FM radio antenna. Or, you can make the
entire radiator length out of regular antenna wire if you wish. The only
critical thing is the total length.

Start out with enough coax so that you will have enough to run up to the
radio from your antenna mounting location.

First, solder the telescoping antenna to the middle conductor on the coax.
Make sure to strip back the shield far enough so that it does not short to
the "radiator" or telescoping part of the antenna. Remember, you can make
the "radiator" entirely out of the center conductor on the coax, so if you
are going to do that, you must make sure the dimensions are correct. Next,
on the other end of the coax, I cut off a piece of coax that is going to
be
the shorting stub. In all cases, I always make the lengths to the long
side.
This way the antenna can be trimmed in to the desired frequency with a SWR
bridge.

To connect the shorting stub to the feedline, carefully remove a small
section of insulation from the center conductor (no more than 1/4 of an
inch). Solder and tape the inner conductor, then join and solder the
shields. To insure proper shielding, split a spare piece of braid
lengthwise
on top of the shield connection, and carefully solder the shield together.
(Rather than disecting the coax cable and soldering, we used a BNC
connector for the shorting stub. We then trimed back for tuning and ended
up with almost no wire needed on the shorting stub. VERY SHORT.)

Don't forget to put a BNC connector on the radio end.

Finally, hang up your antenna in an open space, and tune it in with a SWR
meter. I have found that adjusting the length of the shorting stub has the
most affect in adjusting for minimum SWR but be careful. If you cut too
much
off you will have to re-do it.

Radiator Length: 3' 9.5"
Radiator to shorting stub: 12.9"
Shorting Stub: 1.9" (imagine no BNC connector
when
measuring)

QUESTIONS:

(1) So how do I calculate lengths for other frequencies? ie 121.5MHz,
146.73MHZ, 123.0 MHz
(2) Any chance of damaging my Vertex Standard VX-700 if I start playing
around with this?
(3) Where can I finde a descent but cheap SWR meter?


Thanks,
Carl


I don't know if I am understanding the instructions correctly. I take them
to mean that the 3' 9.5" antenna is attached to the end of the coax and then
you come back away from the base of the antenna 12.9" and splice in a 1.9"
piece of shorted coax. If this is true, then working backwards from the stub
I find that the antenna is about 2.4-j73 Ohms. Seems a bit strange for an
end-fed half wave, but never mind that.

In any case, to adjust the lengths for a new frequency, simply divide 123.45
by the new frequency (in MHz) and then multiply the dimensions by this
number. This is what Richard meant by scaling the antenna for different
frequencies. For example, for 2 meters, you have
123.45/146.73 = 0.8413 so your antenna will be .8413 times 3' 9.5" or 3'
2.3". Adjust the distance from the antenna to the stub and the length of the
stub in the same way.

I recommend that you build your own SWR meter. The one I have in mind will
actually provide protection for your rig while you use it. It is a resistor
bridge type. Read about it he

http://www.qsl.net/xq2fod/Electron/swr/swr.html

He uses two meters, which I would not do (use a switch instead).This circuit
can also be found in the ARRL literature and where it uses just one meter.

Frankly, I agree with other posters here in that this antenna could be very
troublesome for you. Adjusting the shorted stub and its position on the feed
line could have you pulling your hair out. Small errors in dimensions will
have large impacts in SWR. Be sure to look at your SWR when the antenna is
mounted in its new home. Proximity to other objects may have a big effect.

Good luck.

John

Hey, Carl -


"John Smith" wrote in message
...

I don't know if I am understanding the instructions correctly. I take them
to mean that the 3' 9.5" antenna is attached to the end of the coax and
then you come back away from the base of the antenna 12.9" and splice in a
1.9" piece of shorted coax. If this is true, then working backwards from
the stub I find that the antenna is about 2.4-j73 Ohms. Seems a bit
strange for an end-fed half wave, but never mind that.


Oops! I forgot to include the velocity factor of RG58A. Using .73 would make
the antenna 23.9-j199 Ohms. But, this estimate is not really useful to you
and I probably should have left out that whole paragraph. Just ignore it.

John