Antenna informtion needed

[dave]

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]

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]

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

[Vaughn]

"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

[Don Tuite]

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

[Jim Weir]

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

[Jim Weir]

Double whammy with a solidstate output stage. Back in the old vacuum tube days
we just said to hell with the reflective loss, let the RF output tube handle it.
So the plate glowed cherry red for a few seconds, no big deal.

Solid state "plates" don't glow cherry red but for a microsecond or so and that
snap you just heard was $100 worth of frenchfried silicon. So we got clever in
our old age and built in "VSWR Protection Circuits", which do nothing more than
reduce the available power in the transmitter output to something the
transistors can handle. You reduce the transmitted power by reflection and then
reduce it even further by the protection circuit.

Ain't no winnin' with a narrowband antenna and a 760 channel radio.

Jim



Don Tuite >
shared these priceless pearls of wisdom:

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

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

[dave]

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

[Jim Weir]

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

[KJ]

HI Dave- This is a quarter wave whip- must be mounted on metal- shield
to ground and center conductor to whip- the whip is not at ground
potential-

Good luck, KJ

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

[KJ]

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

[dave]

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

[Jim Weir]

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

[UltraJohn]

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
>

[dave]

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