jawod wrote:
...Do I want to go out and correct it or just let the ATU compensate?

I once made a 2000 mile QSO on a light bulb.
--
73, Cecil http://www.w5dxp.com

Owen Duffy wrote:
On Wed, 27 Sep 2006 21:04:42 -0400, jawod wrote:



...Do I want to go out and correct it or just let the ATU compensate?


Can the ATU "compensate" for feedline loss?

Owen
--
My initial question related to a dipole 3 feet too long, no change in
feedline was/is contemplated...consider that a constant.

On Wed, 27 Sep 2006 21:04:42 -0400, jawod wrote:


...Do I want to go out and correct it or just let the ATU compensate?

Can the ATU "compensate" for feedline loss?

Owen
--

On Wed, 27 Sep 2006 22:09:20 -0400, jawod wrote:

Owen Duffy wrote:
On Wed, 27 Sep 2006 21:04:42 -0400, jawod wrote:



...Do I want to go out and correct it or just let the ATU compensate?


Can the ATU "compensate" for feedline loss?

Owen
--
My initial question related to a dipole 3 feet too long, no change in
feedline was/is contemplated...consider that a constant.

The length of feedline(s) may be constant, but the losses in the
feedline(s) depend on VSWR, and nothing you can do with an ATU changes
the loss in feedline(s) beyond the ATU.

I note that you have figured that the dipole is 3' too long, that is
about 3% too long, but the antenna resonance appears about 6% low at
14MHz. Are those two pieces of information consistent?

Owen
--

My initial question related to a dipole 3 feet too long, no change in
feedline was/is contemplated...consider that a constant.


The length of feedline(s) may be constant, but the losses in the
feedline(s) depend on VSWR, and nothing you can do with an ATU changes
the loss in feedline(s) beyond the ATU.

I note that you have figured that the dipole is 3' too long, that is
about 3% too long, but the antenna resonance appears about 6% low at
14MHz. Are those two pieces of information consistent?
Should they be consistent?

Owen
--
Owen,

I get about 4% at 14 MHz, 3.5% at 7 MHz, next to zero on 80M...these are
all relative to the low CW portion of the band where I "live".

I understand your point. VSWR at the XMTR as lowered by the ATU makes
the transmitter happy but the antenna may not be happy or as efficient.

Being a pragmatist and the wet season about to arrive, I just want a
handle on whether, all things considered, it is worthwile to shorten the
antenna or leave it alone.

I've got to start using EZNEC.


John
AB8O

On Wed, 27 Sep 2006 23:36:33 -0400, jawod wrote:



My initial question related to a dipole 3 feet too long, no change in
feedline was/is contemplated...consider that a constant.


The length of feedline(s) may be constant, but the losses in the
feedline(s) depend on VSWR, and nothing you can do with an ATU changes
the loss in feedline(s) beyond the ATU.

I note that you have figured that the dipole is 3' too long, that is
about 3% too long, but the antenna resonance appears about 6% low at
14MHz. Are those two pieces of information consistent?
Should they be consistent?

Yes, the length of open wire section of the feedline influences the
frequencies at which the RG8 VSWR is low, near as much as the dipole
itself.

The band where the "tuning" of the radiator and openwire line section
(together) is most critical is 80m, the optimal bandwidth (from a feed
loss point of view) is narrowest, and has the steepest sides.

However, the place where the VSWR looking into your RG8 is most
predictable is at 20m (typically 14.2MHz) where the common form of the
G5RV should have a three half waves resonant dipole and half wave
electrical open wire section. The Z at the dipole centre will be
around 90+j0, and if the loss on the open wire line is low, the Z into
it will be 90+j0, for a VSWR on the RG8 of just under 2, a little
lower at the tx end.

If you were to find that the VSWR minimises higher or lower than 14.2,
it is a sign that the combination of the dipole length and open wire
section are too long or too short.

If you objective was resonance of the dipole + open wire section at
14.2 (and I now understand that is not your objective), you would be
6% low with your stated observations.

If you are happy with the location of the VSWR dip on 80m, leave it
all alone because the VSWR dip results in the least losses in your
RG8, and if that is of significant length, then the additional losses
are significant. Most other bands are less sensitive than 80m, but
note that a G5RV is not efficient on "all" bands, and so should not
qualify as an all band antenna.

Owen
--

Owen Duffy wrote:
On Wed, 27 Sep 2006 23:36:33 -0400, jawod wrote:


My initial question related to a dipole 3 feet too long, no change in
feedline was/is contemplated...consider that a constant.


The length of feedline(s) may be constant, but the losses in the
feedline(s) depend on VSWR, and nothing you can do with an ATU changes
the loss in feedline(s) beyond the ATU.

I note that you have figured that the dipole is 3' too long, that is
about 3% too long, but the antenna resonance appears about 6% low at
14MHz. Are those two pieces of information consistent?

Should they be consistent?


Yes, the length of open wire section of the feedline influences the
frequencies at which the RG8 VSWR is low, near as much as the dipole
itself.

The band where the "tuning" of the radiator and openwire line section
(together) is most critical is 80m, the optimal bandwidth (from a feed
loss point of view) is narrowest, and has the steepest sides.

However, the place where the VSWR looking into your RG8 is most
predictable is at 20m (typically 14.2MHz) where the common form of the
G5RV should have a three half waves resonant dipole and half wave
electrical open wire section. The Z at the dipole centre will be
around 90+j0, and if the loss on the open wire line is low, the Z into
it will be 90+j0, for a VSWR on the RG8 of just under 2, a little
lower at the tx end.

If you were to find that the VSWR minimises higher or lower than 14.2,
it is a sign that the combination of the dipole length and open wire
section are too long or too short.

If you objective was resonance of the dipole + open wire section at
14.2 (and I now understand that is not your objective), you would be
6% low with your stated observations.

If you are happy with the location of the VSWR dip on 80m, leave it
all alone because the VSWR dip results in the least losses in your
RG8, and if that is of significant length, then the additional losses
are significant. Most other bands are less sensitive than 80m, but
note that a G5RV is not efficient on "all" bands, and so should not
qualify as an all band antenna.

Owen
--
Thanks Owen, for your good advice

John
AB8O

jawod wrote:
I spent the day lowering the folded dipole that I BOUGHT. I was unable
until recently to find how bad the SWR was on most bands. The only
reasonable SWR was on 17 meters.

So, I replaced it with a homemade G5RV, measured appropiately for each
leg of the dipole (14AWG) and for the 300 ohm window line to R/G8U.

I now find reasonable SWR on most bands (WARC, not so much). Here's the
deal: min SWR comes in right at 3.5 MHz, 6.75, 13.43, 18.07 and 29.06.
It appears that I made the classic newbie mistake...antenna too short.

Now, since I am using an ATU, I think this should be close enough for
acceptable efficiency (at least for 80, 40, 20, 17 and 10 meters).

What do you think?

John
AB8O

PS, When I get the time, I'll try EZNEC but for now, I just want to work
what I hear for a change (!)
Update:

Well, yes, the antenna was too long (not too short). I decided to lower
the antenna and shorten each leg by 18 inches. Now it's back up about
45 feet, sloping to about 25 feet. (No change from before.)

Now, SWR dips are at 3.610, 6.710 and 13.580 MHz.

These are not what was expected. Oddly, on 40M SWR dip went further
away from the band edge (as if dipole was lengthened).
I did not change the 300 ohm window line (at 31 feet).

From what I've read here and elsewhere, I should note 80 and 20M
performance as primary considerations.

I'm a little nervous about shortening it further.

John
AB8O

jawod wrote:
Now, SWR dips are at 3.610, 6.710 and 13.580 MHz.

These are not what was expected. Oddly, on 40M SWR dip went further
away from the band edge (as if dipole was lengthened).
I did not change the 300 ohm window line (at 31 feet).

I'll bet your minimum SWR points are not purely resistive.
Any chance of borrowing an MFJ-259B and reporting the
purely resistive points?
--
73, Cecil, http://www.qsl.net/w5dxp

jawod wrote:
I did not change the 300 ohm window line (at 31 feet).

If that's the 300 ohm ladder line with heavy insulation,
it has a VF around 0.8 and Owen's feedline calculator at:

http://www.vk1od.net/tl/tllc.php

says that 1/2WL at 14.2 MHz is ~27.7 feet. If you shorten
your 300 ohm section by about 3 feet, you will have close
to a standard G5RV with the minimum SWR points where they
should be.
--
73, Cecil, http://www.qsl.net/w5dxp