In article ,
(Fred McKenzie) wrote:

In article , ml wrote:

Currently i have a centerfed horiz dipole up pretty high

it's total length is less than 40ft, center feed via my sgc


i was thinking it'd be nice to have a SIGNIFICANT /real order of
magnatude type improvement in 'performance' or gain

ML-

I have a similar question, but I'm not looking for a "real" order of
magnitude improvement. I just want to get on the air.

If I only have room for a short dipole, say 40 feet, and use a tuner such
as the SGC or Icom AH-4, what is the penalty in DB for the lower bands
(160, 75, 60, 40) compared to a half wave on each band?

73,
Fred
K4DII

just a fyi, this really belongs in a new thread

In article , ml wrote:

If I only have room for a short dipole, say 40 feet, and use a tuner such
as the SGC or Icom AH-4, what is the penalty in DB for the lower bands
(160, 75, 60, 40) compared to a half wave on each band?

73,
Fred
K4DII

just a fyi, this really belongs in a new thread

ML-

Perhaps you are right. I was attempting to ask the same question in
reverse: The "penalty" is the opposite of the possible improvement.

So far I've learned that the majority of loss is in the matching network.
Actual gain of a shortened dipole is only about a half dB down, but
radiation resistance is so low that I-squared-R losses in the tuner are
significant.

Someone mentioned that you could reduce losses in the tuner by adding
external loading coils. It seems to me that you would just be moving the
loss out of the tuner into the loading coils.

The question of whether an order of magnitude of improvement can be
attained appears to be answered. If you define that as ten dB of
improvement, then probably not. The total penalty is most likely less
than that.

73,
Fred
K4DII

On Thu, 12 Oct 2006 23:30:17 -0400, (Fred McKenzie)
wrote:


So far I've learned that the majority of loss is in the matching network.
Actual gain of a shortened dipole is only about a half dB down, but
radiation resistance is so low that I-squared-R losses in the tuner are
significant.


You are right that the gain of a short practical dipole is good (and
that requires that the losses are low).

The transmission line is a potential source of high loss when feeding
a short dipole.

There was an article on QST some time back on using a 66' dipole on
all bands. I have written a review which contains some graphs that
cast light on where the losses are, the article is at
http://www.vk1od.net/LOLL/index.htm .

It turns out that for most practical configurations, it is difficult
to achieve good overall efficiency when the dipole length is below
about 35% of a wavelength. You will probably need parallel wire
transmission line rather coax for a shortened dipole.

Someone mentioned that you could reduce losses in the tuner by adding
external loading coils. It seems to me that you would just be moving the
loss out of the tuner into the loading coils.

The question of whether an order of magnitude of improvement can be
attained appears to be answered. If you define that as ten dB of
improvement, then probably not. The total penalty is most likely less
than that.

Taking an "order of magnitude" to mean a tenfold increase or decrease
(10dB), it will be challenging to improve an antenna tenfold by just
adding length unless it was very inefficient in the first place (too
short, lossy feedline + ATU).

If you coax centre fed a 40' dipole on 3.6MHz with 30m of RG58, you
would find opportunity for tenfold improvement by increasing length
(to a particular value).... but only because it was so hopelessly
inefficient as configured.

By my 35% suggestion, a typical parallel line centre fed 40' dipole
will peform ok down to about 8.6MHz. If it is coax fed, it will only
be reasonably efficient on its series resonances (~12MHz, ~36MHz).

Owen
--

On Thu, 12 Oct 2006 23:30:17 -0400, (Fred McKenzie)
wrote:

Someone mentioned that you could reduce losses in the tuner by adding
external loading coils. It seems to me that you would just be moving the
loss out of the tuner into the loading coils.

Hi Fred,

The advice is good, and the logic demands you consider what you say.
What is being exported is the inductance and if you are good at it,
you can build a loading coil that exhibits less loss than the
inductance internal to the tuner.

This reduced loss comes about by larger wire, larger form factor, more
spacing between turns:
Larger wire has more surface area which is significant for skin
effect.
A larger form factor will allow more inductance to replace what is in
the tuner (it will not be a 1:1 even trade, however).
More spacing between wires takes us back to skin effect. Wires that
are in proximity closer than 3 diameters will force currents beneath
the surface of the wires (magnifying the skin effect's loss through
reduced volume for current).

Another virtue is that loading away from the tuner, up in the
structure of the radiator, allows a higher current to flow in that
structure. The general advice is to put the load at least 1/2 up to
2/3rds away from the feedpoint. Outside of this region has
diminishing results. THIS is where you will find gain over a bare
wire and for very short wires, that gain CAN be an order of magnitude.

73's
Richard Clark, KB7QHC

Fred McKenzie wrote:
Someone mentioned that you could reduce losses in the tuner by adding
external loading coils. It seems to me that you would just be moving the
loss out of the tuner into the loading coils.

Very little radiation occurs before the tuner.
Lots of radiation often occurs between the
feedpoint and the loading coils.
--
73, Cecil http://www.w5dxp.com