Antenna informtion needed

If you are talking about one of those old 3/16" stainless steel whips, yes you
do need a connection between the center pin of the RF connector and the element.

You also need a ground plane directly at the base of that antenna if you are
mounting the antenna in a plastic location.

If you plan on using that antenna with a good radio, do yourself a favor and
save your friend a lot of grief by using the antenna as a doorstop. The VSWR at
the band edges with that sucker is something to behold. Remember, that antenna
was designed back in the days when 90 channels (118-127 on 0.1 MHz. spacing) was
just about airline quality.

Jim




dave
shared these priceless pearls of wisdom:

-I have a bent whip antenna that I'm about to mount in a friends Lancair
-and I'm wondering if I should see any connectivity between the center
-conductor of the BNC connector to the stainless steel element?
-
-The ohmmeter shows nothing.
-Is this thing have capacitance coupling?
-
-Thanks
-
-Dave


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

Hey Jim Thanks!

Maybe I should just build him a generic dipole out of wire or copper tape.
How do you get the bandwidth wider, use a wider conductor?

Also, my only vswr meter is a old radioshack one for use in the 10 meter
range, can I retrofit it to work in the 120 mhz area?

Thanks as always

Dave


Jim Weir wrote:

If you are talking about one of those old 3/16" stainless steel whips, yes you
do need a connection between the center pin of the RF connector and the element.

You also need a ground plane directly at the base of that antenna if you are
mounting the antenna in a plastic location.

If you plan on using that antenna with a good radio, do yourself a favor and
save your friend a lot of grief by using the antenna as a doorstop. The VSWR at
the band edges with that sucker is something to behold. Remember, that antenna
was designed back in the days when 90 channels (118-127 on 0.1 MHz. spacing) was
just about airline quality.

Jim




dave
shared these priceless pearls of wisdom:

-I have a bent whip antenna that I'm about to mount in a friends Lancair
-and I'm wondering if I should see any connectivity between the center
-conductor of the BNC connector to the stainless steel element?
-
-The ohmmeter shows nothing.
-Is this thing have capacitance coupling?
-
-Thanks
-
-Dave


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

"dave" wrote in message
news:%10ec.6115$_K3.46405@attbi_s53...
Hey Jim Thanks!

Maybe I should just build him a generic dipole out of wire or copper tape.
How do you get the bandwidth wider, use a wider conductor?

Also, my only vswr meter is a old radioshack one for use in the 10 meter
range, can I retrofit it to work in the 120 mhz area?

It depends, some of those CB vswr meters were pure junk, and some are
OK for the VHF range. The best way to tell is to check it on a couple of
known good installations and also against a dummy load if you have access to
one. If the meter reads in watts, ignore that part; the power will read
high at higher frequencies.

Vaughn

dave
shared these priceless pearls of wisdom:

-Hey Jim Thanks!
-
-Maybe I should just build him a generic dipole out of wire or copper tape.
-How do you get the bandwidth wider, use a wider conductor?

That is correct. The "fatter" the conductor the wider the bandwidth. You can
do a pretty fair job with two or three strands of bare copper wire (stripped
romex) in parallel for each arm of the dipole. The problem is that the fatter
the wire (or the apparent wire) the shorter will be the elements. I know that
½" copper tape centers up pretty nice with a length around 20½". If you use
bare copper, you might cut one of them a half inch longer and one of them half
an inch shorter and see what happens.

-
-Also, my only vswr meter is a old radioshack one for use in the 10 meter
- range, can I retrofit it to work in the 120 mhz area?

9 out of 10 of those old CB VSWR meters aren't worth a darn in the aircraft
band. It is catchascatchcan as to whether yours is going to be "good" or not.
One easy way to tell is to calibrate it with a few known good antenna
installations and see if the CB meter comes even CLOSE to a known good meter.

Jim


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

Ok that brings an interesting thought,
When building a multiband antenna, multiple elements can be attached to
the same feed point and the element cut closest to the resonant freq
will radiate.
I did this with an old HF radio for 20, 40 and 80M.

Is that what you're doing with the three strands of wire?
One say 19.5, the next 20 and the longest at 20.5 inches?
One should be resonant somewhere in the band?

Also, I read that should I use a torid (SP) and put a loop of coax to
keep reflections down.

Sound good?

If I don't have a vswr meter around should I be pretty safe using this
configuration?

Thanks for the help!

Dave


Jim Weir wrote:

dave
shared these priceless pearls of wisdom:

-Hey Jim Thanks!
-
-Maybe I should just build him a generic dipole out of wire or copper tape.
-How do you get the bandwidth wider, use a wider conductor?

That is correct. The "fatter" the conductor the wider the bandwidth. You can
do a pretty fair job with two or three strands of bare copper wire (stripped
romex) in parallel for each arm of the dipole. The problem is that the fatter
the wire (or the apparent wire) the shorter will be the elements. I know that
½" copper tape centers up pretty nice with a length around 20½". If you use
bare copper, you might cut one of them a half inch longer and one of them half
an inch shorter and see what happens.

-
-Also, my only vswr meter is a old radioshack one for use in the 10 meter
- range, can I retrofit it to work in the 120 mhz area?

9 out of 10 of those old CB VSWR meters aren't worth a darn in the aircraft
band. It is catchascatchcan as to whether yours is going to be "good" or not.
One easy way to tell is to calibrate it with a few known good antenna
installations and see if the CB meter comes even CLOSE to a known good meter.

Jim


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

dave
shared these priceless pearls of wisdom:

-Ok that brings an interesting thought,
-When building a multiband antenna, multiple elements can be attached to
-the same feed point and the element cut closest to the resonant freq
-will radiate.
-I did this with an old HF radio for 20, 40 and 80M.

Yes, but you will clearly note that you did not run the elements very close
together, or you might just as well have used a single piece of wire. The
elements were separated, and the more separation the better the multiband
antenna worked.

However, here you are working with a single frequency, and if the wires are
spaced roughly a wire diameter apart, the apparent bandwidth is outrageously
good.



-Also, I read that should I use a torid (SP) and put a loop of coax to
-keep reflections down.

Most folks that use toroids as baluns (actually as chokes) use a toroid that
just slips over the coax and a couple-three of them close to the antenna. This
makes a single-turn coil for each toroid. If you've got the room to use a large
toroid and several loops through it, go for it. Embedding them in the structure
doesn't allow me to do my designs like that {;-)

Don't forget -- the toroid material must be "good" at the frequency of interest,
or all you've done is put a steel nut around the coax, and not a choke.

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

Thanks jim,

Hate to keep this dragging along, but it sounds like multi conductor
ribbon wire might be the ticket for a nice wide band antenna.
Do you have any suggestions for the toroid? I have lots of room for
anything I need.
Is there a good ratshack number or is it not worth worrying about?

Thanks again

Dave


Jim Weir wrote:

dave
shared these priceless pearls of wisdom:

-Ok that brings an interesting thought,
-When building a multiband antenna, multiple elements can be attached to
-the same feed point and the element cut closest to the resonant freq
-will radiate.
-I did this with an old HF radio for 20, 40 and 80M.

Yes, but you will clearly note that you did not run the elements very close
together, or you might just as well have used a single piece of wire. The
elements were separated, and the more separation the better the multiband
antenna worked.

However, here you are working with a single frequency, and if the wires are
spaced roughly a wire diameter apart, the apparent bandwidth is outrageously
good.



-Also, I read that should I use a torid (SP) and put a loop of coax to
-keep reflections down.

Most folks that use toroids as baluns (actually as chokes) use a toroid that
just slips over the coax and a couple-three of them close to the antenna. This
makes a single-turn coil for each toroid. If you've got the room to use a large
toroid and several loops through it, go for it. Embedding them in the structure
doesn't allow me to do my designs like that {;-)

Don't forget -- the toroid material must be "good" at the frequency of interest,
or all you've done is put a steel nut around the coax, and not a choke.

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

dave
shared these priceless pearls of wisdom:

-Thanks jim,
-
-Hate to keep this dragging along, but it sounds like multi conductor
-ribbon wire might be the ticket for a nice wide band antenna.

Don't know, never tried it. I have no idea what the characteristics of the
insulator on ribbon wire is like at VHF, but I suspect it is at least
acceptable, if not good. You would cut the ribbon wire on the diagonal at the
far end of the dipole leaving all the conductors open, and then solder all the
conductors together at the center section of the dipole leg. The diagonal
should be, as I said, 20" at the short point and 21" at the long point from the
center.

How many conductors? Well, we like to make our antennas at least ¼" in rod
diameter or ½" flat tape, so I'd say an inch or so wide with 50% wire fill and
50% insulation fill should be a decent equivalent.

You **will** come back here and let us know how it went, won't you?


-Do you have any suggestions for the toroid? I have lots of room for
-anything I need.

No suggestions for the toroid other than to go to any of the RF ferrite sources
and look for something that will fit and is made of a material that is "good"
(relative term) at 120-140 MHz. Micrometals mix 17 springs quickly to mind.

Do NOT use the Rat Shack "alternator noise" toroid in this application, nor a
toroid salvaged out of a computer power supply. They are LOUSY at VHF.

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

Or put a 50 ohm load on the meter and run your transmitter into it if its
good it will show close to 1:1 vswr then remove the load and run the
transmitter into it (short burst only) and it should show infinite.
John


Jim Weir wrote:

dave
shared these priceless pearls of wisdom:

-Hey Jim Thanks!
-
-Maybe I should just build him a generic dipole out of wire or copper
tape. -How do you get the bandwidth wider, use a wider conductor?

That is correct. The "fatter" the conductor the wider the bandwidth. You
can do a pretty fair job with two or three strands of bare copper wire
(stripped
romex) in parallel for each arm of the dipole. The problem is that the
fatter
the wire (or the apparent wire) the shorter will be the elements. I know
that
½" copper tape centers up pretty nice with a length around 20½". If you
use bare copper, you might cut one of them a half inch longer and one of
them half an inch shorter and see what happens.

-
-Also, my only vswr meter is a old radioshack one for use in the 10 meter
- range, can I retrofit it to work in the 120 mhz area?

9 out of 10 of those old CB VSWR meters aren't worth a darn in the
aircraft
band. It is catchascatchcan as to whether yours is going to be "good" or
not. One easy way to tell is to calibrate it with a few known good antenna
installations and see if the CB meter comes even CLOSE to a known good
meter.

Jim


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

On Sat, 10 Apr 2004 16:19:25 -0700, Jim Weir wrote:

The VSWR at
the band edges with that sucker is something to behold. Remember, that antenna
was designed back in the days when 90 channels (118-127 on 0.1 MHz. spacing) was
just about airline quality.

Respectful, question, Jim: If he has a transistor output stage. . . .
(etc. etc.)

Don