that about sums it up.

stub matching looks attractive, but how do I actually build one of
these suckers? Do I have to carve out a piece of coax and then fit
another one into it, if that makes sense? Won't that be nearly
impossible to solder?

physically, it would be easier to make a stub section with sections of
single wire line, but I don't know how to find Z0 of my 14 gauge wire?

Is stub matching not the answer?

The antenna is designed for about 450 Mhz, if that helps anyone.

Thanks for your help.

wrote:
that about sums it up.

stub matching looks attractive, but how do I actually build one of
these suckers? Do I have to carve out a piece of coax and then fit
another one into it, if that makes sense? Won't that be nearly
impossible to solder?

physically, it would be easier to make a stub section with sections of
single wire line, but I don't know how to find Z0 of my 14 gauge wire?

The ARRL Handbook and Antenna Book contain the formulas.

Is stub matching not the answer?

How about replacing the stub with a capacitor.

The antenna is designed for about 450 Mhz, if that helps anyone.

At a point on the 50 ohm coax that is 8.75"*VF from
the 190+j140 ohm load, install a 14.2 pF cap to ground.
You can do it with a stub but a cap is much easier.
--
73, Cecil http://www.w5dxp.com

On Sat, 8 Dec 2007 06:58:33 -0800 (PST), wrote:

that about sums it up.

stub matching looks attractive, but how do I actually build one of
these suckers?

Using two lengths of transmission line that you have to cut to length,
or build them so that they slide like a trombone.

Do I have to carve out a piece of coax and then fit
another one into it, if that makes sense? Won't that be nearly
impossible to solder?

The trombone line will probably more successful, although more
difficult to accomplish. You would build it out of brass, staged
tubing (4 diameters), with BNC connectors at each end. All quite
complex as I said, but it would suit a far wider range of matching
applications. You would also need a shorted BNC to terminate one of
the trombones so that you had that variation covered too. Use a T
connector set a the end of the drive line, one trombone line goes to
the load, and the other goes to the remaining T connection. You might
be able to get away with this last trombone section and just use the T
and a short run of standard coax to the load - emphasis on "might."
These two trombone lengths should have the capacity to vary over a
range at least slightly greater than half a wavelength. You will want
to build them longer for mechanical reasons. The sliding contacts
will not be very reliable (professional tuners have spring loaded
surfaces).

Welcome to the wonderful world of double stub tuning.

Gamma matching at the antenna would make this vastly easier.

73's
Richard Clark, KB7QHC

trombone section looks a little out of my league. I'm kind of in a
hurry. It's kind of too late to gamma match I think because I've
already soldered a connector onto the end of the antenna (its helical,
so i can't just reach in there and fool with it much, you know?)

It's tempting to just throw a cap in there, but will a discrete
capacitor look as it should at 450 Mhz? Or should I make it out of a
transmission line?

I did not know that books documented the Z0 of single wire conductors,
I'll look into that, thanks. BTW what is the full title of the APRL?
I hear it often (I'm kind of a rookie)

THanks everyone

if you have an instrument that is good enough to read the impedance like
that just try experimental values... a foot of small coax is around 20pf,
unfortunately that is a significant fraction of a wavelength so it will act
more like an open stub than a pure capacitor. try just putting a foot in
parallel with the feedpoint and snipping it shorter a bit at a time to see
if it helps.


wrote in message
...
trombone section looks a little out of my league. I'm kind of in a
hurry. It's kind of too late to gamma match I think because I've
already soldered a connector onto the end of the antenna (its helical,
so i can't just reach in there and fool with it much, you know?)

It's tempting to just throw a cap in there, but will a discrete
capacitor look as it should at 450 Mhz? Or should I make it out of a
transmission line?

I did not know that books documented the Z0 of single wire conductors,
I'll look into that, thanks. BTW what is the full title of the APRL?
I hear it often (I'm kind of a rookie)

THanks everyone

I'm still deciding etc. One question that is a bit off topic:

at this website

http://emc.ice.uec.ac.jp/~xiao/Wire/index.html

should I use the permittivity of the insulator of the wire im using?
or air? which is more significant?

On Sat, 8 Dec 2007 09:24:30 -0800 (PST), wrote:

trombone section looks a little out of my league. I'm kind of in a
hurry. It's kind of too late to gamma match I think because I've
already soldered a connector onto the end of the antenna (its helical,
so i can't just reach in there and fool with it much, you know?)

To put it mildly, you are in a world of hurt.

As Dave points out elsewhere, if you have the instrumentation to
measure to the implied accuracy (or at least resolution) as is
suggested in this subject line; then you should cut-and-try.

As for capacitors, don't even think of anything but surface mount. But
where? You are back into that world of hurt if you don't have the
time to do it right (a simple gamma match).

It could be as easy, however, as crushing or spreading the first
couple of turns of the helix, or playing with the proximity of the
ground plane, or both. If you don't have time, then you certainly
shouldn't be playing with caps.

73's
Richard Clark, KB7QHC

On Dec 8, 1:06 pm, Richard Clark wrote:
On Sat, 8 Dec 2007 09:24:30 -0800 (PST), wrote:
trombone section looks a little out of my league. I'm kind of in a
hurry. It's kind of too late to gamma match I think because I've
already soldered a connector onto the end of the antenna (its helical,
so i can't just reach in there and fool with it much, you know?)

To put it mildly, you are in a world of hurt.

As Dave points out elsewhere, if you have the instrumentation to
measure to the implied accuracy (or at least resolution) as is
suggested in this subject line; then you should cut-and-try.

As for capacitors, don't even think of anything but surface mount. But
where? You are back into that world of hurt if you don't have the
time to do it right (a simple gamma match).

It could be as easy, however, as crushing or spreading the first
couple of turns of the helix, or playing with the proximity of the
ground plane, or both. If you don't have time, then you certainly
shouldn't be playing with caps.

73's
Richard Clark, KB7QHC

Ok. again I'm not that experienced. What exactly is a gamma match?

On Sat, 8 Dec 2007 10:41:47 -0800 (PST), wrote:

Ok. again I'm not that experienced. What exactly is a gamma match?

Your time would be better served in pursuing the simpler physical
manipulations of the existing design.

If you have more time, research google with the keywords
gamma match helix antenna.

Telling you "what" it is would be less productive to simply looking at
examples as I perceive you are results oriented. This is an end of
quarter requirement, isn't it?

73's
Richard Clark, KB7QHC

yep its an end of the semester project. BUT, we never learned anything
about impedance matching, so I feel no shame in asking people like
yourself. Thanks for your help.

I'm at home now, and from what I can tell, I can get a good reactive
match by changing the length of the wire from the ground plane to the
connector? (while keeping the wire from the helix to the connector
constant? I think I will try this.

The real part I'm not too worried about.