unable to tune doublet at higher freq.
 

I have a 135' long doublet, 45' in the air fed with 450 ohm ladderline. It
tunes fine and works well through an MFJ tuner on all bands up to 21MHz.
Above 21MHz I have a very difficult time tuning the antenna to get any SWR
below 3:1. I cut 6' off the feedline when it was finally dressed into my
shack and that made it worse. I have now reinstalled the extra 6' and that
is better for tuning, but not yet perfect. I guess I'll just have to keep
adding or subtracting small amounts of feedline to get it right?

If so, are we talking inches or what? Any ideas appreciated.

Larry VE7EA

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to reply via email remove "fake"

Larry
You're trying to feed the antenna at a current node.
Change the length of the antenna, not the feed.
Try asymmetric legs.
73
H.
NQ5H

"Larry Gagnon" wrote in message
om...
I have a 135' long doublet, 45' in the air fed with 450 ohm ladderline. It
tunes fine and works well through an MFJ tuner on all bands up to 21MHz.
Above 21MHz I have a very difficult time tuning the antenna to get any SWR
below 3:1. I cut 6' off the feedline when it was finally dressed into my
shack and that made it worse. I have now reinstalled the extra 6' and that
is better for tuning, but not yet perfect. I guess I'll just have to keep
adding or subtracting small amounts of feedline to get it right?

If so, are we talking inches or what? Any ideas appreciated.

Larry VE7EA

--
********************************
to reply via email remove "fake"

Where is it fed? [center, end, off center, etc.]

If center fed, the impedance looking into the feedline exceeds the
tuning range of your tuner. I would start by adding a little less than
1/4 wavelength at 28.3 MHz of additional feedline [something around 6 feet].

I use a CF doublet with 600 Ohm tuned feeders and an OLD [OLD] Murch
UT2000A tuner with a very good balun.

+ + +

Larry Gagnon wrote:
I have a 135' long doublet, 45' in the air fed with 450 ohm ladderline. It
tunes fine and works well through an MFJ tuner on all bands up to 21MHz.
Above 21MHz I have a very difficult time tuning the antenna to get any SWR
below 3:1. I cut 6' off the feedline when it was finally dressed into my
shack and that made it worse. I have now reinstalled the extra 6' and that
is better for tuning, but not yet perfect. I guess I'll just have to keep
adding or subtracting small amounts of feedline to get it right?

If so, are we talking inches or what? Any ideas appreciated.

Larry VE7EA

Above 21MHz I have a very difficult time tuning the antenna to get any SWR
below 3:1.

Hi Larry, My good friend Reg Edwards pointed out some time ago that most
antenna tuners give it up above 21 MHZ. This is due to stray Inductance and
Capacitance in the Garden Variety Antenna Tuner. Makes it real difficult to
tune certain antennas above 21 MHZ. As others have mentioned, change antenna
length, or feed length and see what that does. There are programs that will
determine correct tuner settings for a given impedance, I prefer the Smith
Chart.
If you can determine the Impedance, R+/-jx of the feed point, there are a
number of alternative ways of designing a matching network.
73 Gary N4AST

Larry
Add 3 feet of wire to one end of the doublet and see what happens.
H.

"H. Adam Stevens, NQ5H" wrote in message
link.net...
Larry
You're trying to feed the antenna at a current node.
Change the length of the antenna, not the feed.
Try asymmetric legs.
73
H.
NQ5H

"Larry Gagnon" wrote in message
om...
I have a 135' long doublet, 45' in the air fed with 450 ohm ladderline.
It
tunes fine and works well through an MFJ tuner on all bands up to 21MHz.
Above 21MHz I have a very difficult time tuning the antenna to get any
SWR
below 3:1. I cut 6' off the feedline when it was finally dressed into my
shack and that made it worse. I have now reinstalled the extra 6' and
that
is better for tuning, but not yet perfect. I guess I'll just have to
keep
adding or subtracting small amounts of feedline to get it right?

If so, are we talking inches or what? Any ideas appreciated.

Larry VE7EA

--
********************************
to reply via email remove "fake"

I have a 135' long doublet, 45' in the air fed with 450 ohm ladderline. It
tunes fine and works well through an MFJ tuner on all bands up to 21MHz.
Above 21MHz I have a very difficult time tuning the antenna to get any SWR
below 3:1. I cut 6' off the feedline when it was finally dressed into my
shack and that made it worse. I have now reinstalled the extra 6' and that
is better for tuning, but not yet perfect. I guess I'll just have to keep
adding or subtracting small amounts of feedline to get it right?

Yes, try adding some feedline.

I have a ~120' doublet center fed with ladderline. No problem matching it
on 80/40/20/15/10 with a Johnson Matchbox. But I did have to change the
feedline length from what I started with.

Torsten
N4OGW