Antenna informtion needed

dave wrote in message news:n1%dc.5551$_K3.46068@attbi_s53...
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

HI Dave- This is a quarter wave whip- Must be mounted on metal- shield
to ground next to whip- center conducter to whip- The whip is not at
ground potential- Good luck- KJ

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

Well I built the antenna with four conductor ribbon cable. The total
width of the cable is about 1/3 - 1/2".
I would have liked it wider, but it's all I had around.

The longest conductor was 21" and each one was shortened about 1/3"
until the shortest one was 20 inches.
Didn't have a Toroid to use for a balum, i can put that on later.

We taped it to the side of the Fuselage for a test run and used a
signal/field strength meter to test for output.

The meter peaked around 124.75 MHZ. Hopefully this is telling me that
the antenna is resonant around that frequency.
The antenna was more in the horzontal than in the vertical plane.

Dallas Air Park, where the airplane is parked, is about three nm from
Addison airport which is tower controlled. I made a call to Addison
ground control and they could hear me. WOW! Not bad for inside a hangar
and a few miles away.

When I get a VSWR meter I'll take a reading and really see how close I am.

Thanks all for the help.

Cheers

Dave


Jim Weir wrote:
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