Still having fun playing with my 8.5 foot whip on the metal patio roof, with
coax feedline to an antenna tuner in the shack. Operating on 15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and move
around the SWR circle on the Smith chart. Apparently this provides a set of
matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess is
that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to be
more difficult to match, and should be avoided?

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Try rearranging your network, try pi series C, pi shunt C, etc.

Also make friends with someone with a vswr analyzer ;-)

On Jan 2, 3:39*am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio
roof, with coax feedline to an antenna tuner in the shack.
I was unable to get a match on 15 meters with the antenna tuner.

EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR
of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms,
depending on feedline length. Not many tuners will match 1.25 ohms
with reasonable efficiency. In this case, a feedline length of an
integer number of wavelengths (N*30') plus 13 feet would probably
allow for a reasonable tuner match for VF=0.66 coax if you can
tolerate the considerable feedline losses.
--
73, Cecil, w5dxp.com

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Have you ever compared the field strengths above and below the steel
cover?

"Wond" wrote in message ...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Have you ever compared the field strengths above and below the steel
cover?
-
No I haven't. What would be the practical tests? Example, measuring FS at
x feet from the antenna at y height above the cover, vs below the cover at
what point?

"dave" wrote in message
...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Try rearranging your network, try pi series C, pi shunt C, etc.

Also make friends with someone with a vswr analyzer ;-)
-
Or, I could leave in the additional feedline ;-)
But a good point is that I have an old homebrew 2 element tuner with
alligator clip configuration. Might give that a try just for fun.

I have one of the older MFJ analyzers, but I wouldn't trust the accuracy at
high VSWRs.

"W5DXP" wrote in message
...

On Jan 2, 3:39 am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio
roof, with coax feedline to an antenna tuner in the shack.
I was unable to get a match on 15 meters with the antenna tuner.

EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR
of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms,
depending on feedline length. Not many tuners will match 1.25 ohms
with reasonable efficiency. In this case, a feedline length of an
integer number of wavelengths (N*30') plus 13 feet would probably
allow for a reasonable tuner match for VF=0.66 coax if you can
tolerate the considerable feedline losses.
--
73, Cecil, w5dxp.com
-
Thanks for the comments. When I used EZNEC I didn't know what to use for
ground, so I used Real/Minimec. I'm showing about 18-j142 as the feedpoint
impedance at 21.1 Mhz, which is not totally out of the ballpark with your
number.

At any rate, I have bought enough telescoping tubing sections to build
about a 20 foot vertical. Final height is subject to approval by "the
Admiral". Fabrication of a more substantial base support will be necessary.

In the meantime, this AM I replaced the 8.6 ft whip with about 13 feet of
telescoping tubing, using the same mobile whip mount clamped to conduit.
Resonance is around
18.7 MHz and it loads up nicely on 18 through 10, with 10 meters having the
highest calculated swr of 17.

With the antenna removed, the feedline shows a 1/4 wave null at 4.33MHz.
This gives an estimated 1/2 wave electrical length of 57 feet at 8.66 MHz,
or with 0.66 VF a physical length of 37.6 ft. That seems about right.

I'll get a Smith chart and see where the existing feedline moves the
parameters. Per your comments I'll try to avoid low r values.

Wish the bands would open back up like they were in Oct and Nov.
-
Wayne
W5GIE

On Mon, 02 Jan 2012 08:47:29 -0800, Wayne wrote:

"Wond" wrote in message ...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:


Have you ever compared the field strengths above and below the steel
cover?
-
No I haven't. What would be the practical tests? Example, measuring FS
at x feet from the antenna at y height above the cover, vs below the
cover at what point?

I was thinking, it would be a rough indication what proportion of
power is radiated from the feedline.

"Wond" wrote in message ...

On Mon, 02 Jan 2012 08:47:29 -0800, Wayne wrote:

"Wond" wrote in message ...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:


Have you ever compared the field strengths above and below the steel
cover?
-
No I haven't. What would be the practical tests? Example, measuring FS
at x feet from the antenna at y height above the cover, vs below the
cover at what point?

I was thinking, it would be a rough indication what proportion of
power is radiated from the feedline.
-
Good point, and I'm not sure that the feedline is currently in optimal
configuration.

The feedline lies on the metal roof to the edge, where there is a small 8
turn loop. Then it goes another 6-8 feet to the tuner.

My thoughts are that your antenna theory has holes in it.
If you want it to tune on 15 meters, it needs to be a fraction of a wavelength long on 15 meters - which a antenna tuned for 10 meters has a real hard time sometimes tuning up on 12 or 15 meters.

Instead of trying to redesign the mouse trap, why not invest your money towards a used 11 meters chicken band antenna.
A antenna - regardless of who makes it or what they market it for - is a antenna.

By shortening a 11 meter antenna a couple of inches, it will work wonderfully for 10 meters.

The suggested length of coax with solid dielectric .66 velocity factor usually comes out somewhere around 18 feet long for a mobile, so working with the Smith Chart will get your SWR - line length to a better SWR, but to make it resonant - it needs to be a certain size.

You would be better off designing the antenna for 15 meters and then using the transmatch to tune it down to 10 meters then to try to use a 10 meters antenna and try to tune it up for 15. Then again, your antenna really isn't any good for either band in my opinion, due to the fact that a 5/8ths wave antenna probably works best in most situations for a base station radio for 10 meters and your antenna is way too short.

Even a 1/4 wave antenna at 15 meters would probably need to be at least 11.5 feet long, with a nominal length of about 31 feet using a vertical antenna.