On Thu, 12 Oct 2006 14:42:39 -0400, (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.

Hi Fred,

Good, simple antennas are not going to show an "order of magnitude"
improvement under any circumstance short of issues related to loss.

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?

What do the manufacturers say? OK, they probably guarantee nothing
other than a match (I am familiar with the local manufacturer's claim
of loading a wet string.)

The radiation resistance of a 40' dipole in the 160M band is 1 Ohm.
The contribution of loss resistance in proportion to this reveals
efficiency/loss. If anything in those boxes contributes as much as 1
Ohm, then you are down by 3dB before the signal hits the airways.

The only question is can that additional 1 Ohm contribution dissipate
the heat without self destruction? Think about it, 50W of heat inside
that box. The manufacturer would probably guess correctly that you
would never feel it, unless you are working RTTY.

An additional 1 Ohm attributable to the manufacturer's designs may be
an outlandish assertion. Again, consult their specifications.

73's
Richard Clark, KB7QHC

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

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?

Some quasi-wild guesses for a 40 foot dipole relative to full size at
the same height, probably best case.

160m: -16dB
75m: -8dB
60m: -3dB
40m: -1.5dB

These numbers don't mean much. I used EZNEC to look at the losses in
Q=100 loading coils with enough reactance to bring thing to resonance
on each band. No attempt to match to 50 ohms was made, but I made the
Q kind of low to reflect other tuner losses. Still, these are almost
certainly optimistic, especially on 160.

Dan

On 12 Oct 2006 23:32:20 -0700, "
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?

Some quasi-wild guesses for a 40 foot dipole relative to full size at
the same height, probably best case.

160m: -16dB
75m: -8dB
60m: -3dB
40m: -1.5dB

These numbers don't mean much. I used EZNEC to look at the losses in
Q=100 loading coils with enough reactance to bring thing to resonance
on each band. No attempt to match to 50 ohms was made, but I made the
Q kind of low to reflect other tuner losses. Still, these are almost
certainly optimistic, especially on 160.

Dan, looking at a hypothetical unloaded scenario...

I don't know what feedpoint Z you got for a centre fed 40' dipole at
7MHz... but lets guess somewhere about 11+j750. Lets feed the antenna
with 15m (~50') of RG58C/U for a loss of 18dB (efficiency 1.6%) and an
input z at the ATU of 44-j179. The ATU should handle that with very
low loss... so overall, the efficiency is ~1.6%.

You can do these calcs with the line loss calculator at
http://www.vk1od.net/tl/tllc.php, nothing to unzip!

Adding 26' of wire to the dipole should improve the antenna to better
than 80% efficiency or a fifty fold improvement on transmit. But it
only worked so dramatically because the 40' dipole centre fed with
coax is so inefficient.

Owen
--

On Fri, 13 Oct 2006 06:51:50 GMT, Owen Duffy wrote:

7MHz... but lets guess somewhere about 11+j750. Lets feed the antenna

Spellcheckers aren't smart enough, should read:

7MHz... but lets guess somewhere about 11-j750. Lets feed the antenna
--

Owen Duffy wrote:
On Fri, 13 Oct 2006 06:51:50 GMT, Owen Duffy wrote:

7MHz... but lets guess somewhere about 11+j750. Lets feed the antenna

Spellcheckers aren't smart enough, should read:

7MHz... but lets guess somewhere about 11-j750. Lets feed the antenna

--

Hahaha... guess you need a complex conjugation checker. Owen, I was
assuming no feedline and an antenna tuner at the feedpoint and being
very gracious towards that antenna tuner's abilities ( I wouldn't be
surprised if a tuner could match 11-j750 with 1.5dB of loss, though
maybe not a 100W-class autotuner).

As far as the transmission line calculator goes, I use it ALL the time.
It's a fantastic resource, both technically and ... well,
rhetorically. A lot of folks will say their antenna works even if their
antenna is a 40 foot dipole fed with 50 feet of RG-58 on 40m.

They can hear people with it, their rig's autotuner loads it up just
fine on 40m, and they can "make contacts" but their effective radiated
power is going to be 1.6 watts... and feedline loss is an
environmentally-unfriendly way to be a QRP operator.

Dan