Been cleaning out the garage this weekend and came across an old attempt at
build a 432Mhz helix antenna. As I remember I gave up on it because I was
having trouble building the matching section which was a 11/4wl long fin
that attches to the helix as it apprached the feed point. It was proving
difficult for me to fabricate. I was wondering if the fin could not be
attached to the counterpoise instead of the helix, adjusting the space
beteen the fin and the helix to get a match.

Jimmie.

"Jimmie D" wrote in message
...
Been cleaning out the garage this weekend and came across an old attempt
at build a 432Mhz helix antenna. As I remember I gave up on it because I
was having trouble building the matching section which was a 11/4wl long
fin that attches to the helix as it apprached the feed point. It was
proving difficult for me to fabricate. I was wondering if the fin could
not be attached to the counterpoise instead of the helix, adjusting the
space beteen the fin and the helix to get a match.

Jimmie.

One way is to run a single wire next to the reflector from the feedpoint to
the transmission line. If the Z0 of the helix is 120 Ohms, space the wire
from the refleector so that at that end it forms an impedance of 120 Ohms.
Space it at the coax end so the impedance is 50 Ohms. I think that
theoretically the impedance is supposed to change is some logarithmic
fashion, but linear reduction of distance works. If the reflectopr is not
solid, fasten a metal strip to it to get a smooth impedance. I can't
remember the minimum length for the conversion line; something like a
wavelength. Google helix antenna matching.

Tam/WB2TT

"Jimmie D" wrote in message
...
Been cleaning out the garage this weekend and came across an old attempt
at build a 432Mhz helix antenna. As I remember I gave up on it because I
was having trouble building the matching section which was a 11/4wl long
fin that attches to the helix as it apprached the feed point. It was
proving difficult for me to fabricate. I was wondering if the fin could
not be attached to the counterpoise instead of the helix, adjusting the
space beteen the fin and the helix to get a match.

Jimmie.


Hi Jimmie

AMSAT Journal, November/December 2005, published an article authored by
Clare Fowler, VE3NPC, related to matching axial mode helix antennas.
If you want a 'scanned copy' contact me at your convenience.

Jerry

"Tam/WB2TT" wrote in message
. ..

"Jimmie D" wrote in message
...
Been cleaning out the garage this weekend and came across an old attempt
at build a 432Mhz helix antenna. As I remember I gave up on it because I
was having trouble building the matching section which was a 11/4wl long
fin that attches to the helix as it apprached the feed point. It was
proving difficult for me to fabricate. I was wondering if the fin could
not be attached to the counterpoise instead of the helix, adjusting the
space beteen the fin and the helix to get a match.

Jimmie.

One way is to run a single wire next to the reflector from the feedpoint
to the transmission line. If the Z0 of the helix is 120 Ohms, space the
wire from the refleector so that at that end it forms an impedance of 120
Ohms. Space it at the coax end so the impedance is 50 Ohms. I think that
theoretically the impedance is supposed to change is some logarithmic
fashion, but linear reduction of distance works. If the reflectopr is not
solid, fasten a metal strip to it to get a smooth impedance. I can't
remember the minimum length for the conversion line; something like a
wavelength. Google helix antenna matching.

Tam/WB2TT

Pardon my ignorance but how can I work out what the Z0 of an antenna is?
As you say above, "If the Z0 of the helix is 120 Ohms".

I'm not totally stupid but could never work it out.
It isn't as simple as connecting an ohm meter to it......is it ?

Boozo.

Jimmie D wrote:
Been cleaning out the garage this weekend and came across an old attempt at
build a 432Mhz helix antenna. As I remember I gave up on it because I was
having trouble building the matching section which was a 11/4wl long fin
that attches to the helix as it apprached the feed point. It was proving
difficult for me to fabricate. I was wondering if the fin could not be
attached to the counterpoise instead of the helix, adjusting the space
beteen the fin and the helix to get a match.

I've always had success matching a helix by running the first 1/4 turn
(starting at the feed point) parallel to the reflector and changing the
spacing between the 1/4 turn and the reflector until I get a low SWR.
If the reflector is mesh I put a piece of sheet metal on the mesh under
that 1/4 turn. It's worked every time, no fins, no 1/4 wave sections of
coax, etc. For my 432 MHz helix it ended up with about 1/4 inch
spacing, using #10 solid wire for the helix.

-W8LNA

"gwatts" wrote in message
...
Jimmie D wrote:
Been cleaning out the garage this weekend and came across an old attempt
at build a 432Mhz helix antenna. As I remember I gave up on it because I
was having trouble building the matching section which was a 11/4wl long
fin that attches to the helix as it apprached the feed point. It was
proving difficult for me to fabricate. I was wondering if the fin could
not be attached to the counterpoise instead of the helix, adjusting the
space beteen the fin and the helix to get a match.

I've always had success matching a helix by running the first 1/4 turn
(starting at the feed point) parallel to the reflector and changing the
spacing between the 1/4 turn and the reflector until I get a low SWR. If
the reflector is mesh I put a piece of sheet metal on the mesh under that
1/4 turn. It's worked every time, no fins, no 1/4 wave sections of coax,
etc. For my 432 MHz helix it ended up with about 1/4 inch spacing, using
#10 solid wire for the helix.

-W8LNA

Thats what I did yesterday sort of. I. took the piece of metal I was going
to fasten to the helix and fastened it to the reflector and then adjusted
the spacing for best SWR. The difference is I moved the counterpoise instead
of the antenna to get the match. I do see why others have opted to put the
fin on the wire. The spacing is so close doing it the ways we did that you
may get arcing when using high power. If I ever build another one I will
probably use copper instead of Al tubing for the antenna and solder the fin
on the antenna.

Jimmie

Jimmie D wrote:
"gwatts" wrote in message
...

Jimmie D wrote:

Been cleaning out the garage this weekend and came across an old attempt
at build a 432Mhz helix antenna. As I remember I gave up on it because I
was having trouble building the matching section which was a 11/4wl long
fin that attches to the helix as it apprached the feed point. It was
proving difficult for me to fabricate. I was wondering if the fin could
not be attached to the counterpoise instead of the helix, adjusting the
space beteen the fin and the helix to get a match.

I've always had success matching a helix by running the first 1/4 turn
(starting at the feed point) parallel to the reflector and changing the
spacing between the 1/4 turn and the reflector until I get a low SWR. If
the reflector is mesh I put a piece of sheet metal on the mesh under that
1/4 turn. It's worked every time, no fins, no 1/4 wave sections of coax,
etc. For my 432 MHz helix it ended up with about 1/4 inch spacing, using
#10 solid wire for the helix.

-W8LNA


Thats what I did yesterday sort of. I. took the piece of metal I was going
to fasten to the helix and fastened it to the reflector and then adjusted
the spacing for best SWR. The difference is I moved the counterpoise instead
of the antenna to get the match. I do see why others have opted to put the
fin on the wire. The spacing is so close doing it the ways we did that you
may get arcing when using high power. If I ever build another one I will
probably use copper instead of Al tubing for the antenna and solder the fin
on the antenna.

You might be misunderstanding me, I didn't attach the piece of sheet
metal to the mesh as a fin but flat onto the mesh so what I ended up
with is a wire over a smooth, flat surface for a matching section.

- W8LNA

"Boozo" wrote in message
...

"Tam/WB2TT" wrote in message
. ..

"Jimmie D" wrote in message
...
Been cleaning out the garage this weekend and came across an old attempt
at build a 432Mhz helix antenna. As I remember I gave up on it because I
was having trouble building the matching section which was a 11/4wl long
fin that attches to the helix as it apprached the feed point. It was
proving difficult for me to fabricate. I was wondering if the fin could
not be attached to the counterpoise instead of the helix, adjusting the
space beteen the fin and the helix to get a match.

Jimmie.

One way is to run a single wire next to the reflector from the feedpoint
to the transmission line. If the Z0 of the helix is 120 Ohms, space the
wire from the refleector so that at that end it forms an impedance of 120
Ohms. Space it at the coax end so the impedance is 50 Ohms. I think that
theoretically the impedance is supposed to change is some logarithmic
fashion, but linear reduction of distance works. If the reflectopr is not
solid, fasten a metal strip to it to get a smooth impedance. I can't
remember the minimum length for the conversion line; something like a
wavelength. Google helix antenna matching.

Tam/WB2TT

Pardon my ignorance but how can I work out what the Z0 of an antenna is?
As you say above, "If the Z0 of the helix is 120 Ohms".

I'm not totally stupid but could never work it out.
It isn't as simple as connecting an ohm meter to it......is it ?

Boozo.

Yoou need to find design equations for it. I think it was a function of
helix diameter and pitch. It is in either the Radio Amateurs Handbook, or
the ARRL Antenna book. Probably any book on antennas. Info should also be
available on the 'net.

You could measure the SWR. You know it is more than 50 Ohms; so, if the SWR
is 3:1, that means 150 Ohms rather than 17.

Tam

Tam, WB2TT wrote:
"It is in either the Radio Amateurs Handbook, or the ARRL Antenna Book."

My ARRL Antenna Book is the 19th edition. Feed-point assembly and
matching are diagrammed on page 19-32 for a 435 MHz axial-mode helix.

Best regards, Richard Harrison, KB5WZI