Jeff Liebermann wrote:

snip

Somewhere on my computah is a series of models that I built for a
simple 20 meter dipole, that started with an idealized free space
model, and progressed towards a real installation which by coincidence
resembles my house. I threw in everything that might have an effect
on the pattern to see what might happen. I got stuck at including the
sloping hillside because NEC2 seems to only include a flat earth. I've
also done similar studies for commercial antennas mounted on very real
and quite cluttered towers. Doing these incrementally is an excellent
introduction into the difference between ideal antenna patterns, as
found in the literature, and the nightmarish reality of real antenna
installations.

Yes, this is another thing typically ignored by amateurs.

There are lots of plans out there for J-poles made of pipe where one
of the legs is extended at the bottom to provide the mounting, typically
by bolting it to a mast. The assumption is there is no current in that
bottom leg.

If you model that you find that there really is current in that bottom
leg and it can REALLY screw up the antenna characteristics.



--
Jim Pennino

John S wrote:
On 10/11/2014 12:51 PM, wrote:
John S wrote:

snip

OK, so lets analyze my results:

Conditions are free space, wire is #14 gauge but may have zero ohms
where noted. The antenna is a dipole with the source connected at the
center, F=7MHz. I'm using EZNEC with a source of 1 watt. Antenna
resonance plays no part in this. # segments = 99 unless otherwise noted.

Lambda Wire Rin Gavg(dbi) Gmax(dbi) Efficiency

0.5 zero 80 0 2.16 100%
0.5 #14 73.6 -.09 2.08 98%

0.25 zero 13.2 0 1.85 100%
0.25 #14 13.7 -.17 1.69 96%

0.125 zero 3 0 1.78 100%
0.125 #14 3.25 -.33 1.45 93%

0.05 zero .464 0 1.76 100%
0.05 #14 .556 -.78 0.98 83%

Rin is the terminal resistance only. Gave is the average gain integrated
over the pattern, Gmax is the highest gain detected.

Unless I have done something wrong, I see that a dipole that is .05
wavelengths long is within 20% of being as efficient as a half-wave
dipole. Even including wire resistance.

I invite discussion in any case.

The diameter of #14 solid wire is 0.0641"; how about a line for #8, which
is 0.1285"?


0.05 #8 0.515 -.41 1.36 91.1%


Free space, no ground loss.

So it seems that a .05 lamda dipole is only about 7% less efficient than
a full size dipole wit suitable wire...

So much for "short antennas are not efficient".

Now, if we could just invent room temperature superconductor wire and
lossles toroids we could match the thing to a 50 Ohm system...


--
Jim Pennino

wrote in message
...
I've done a lot of modeling with V's and inverted V's. Except for a slight
diffenence due to ground effects at low heights, there is no difference
between them.

As the angle goes from 180 degrees, i.e. a dipole, the impedance goes
down and and the pattern spreads out.


I have not played with the programs , but often wondered what the effect is
on antennas that are suported on the ends and the middle is dropped in a U
or V shape and not the inverted V shape. Many antenna books and articals
seem to show the flat top and inverted V paterns, but I have not seen any
with the actual V type.



---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

On 10/12/2014 1:06 PM, wrote:
Lostgallifreyan wrote:
John S wrote in :

Something else might be interesting; include the effects of sag
(centenary) in a wire antenna.

Agreed. I was thinking about that possibility last night. Meaning 'catenary',
perhaps? As in 'hanging chain'? I doubt any longwire would lack this, so
modelling it would be useful.

I doubt you will see any significant difference.


Do you have any data or a simulation which supports your position?

(snip irrelevant portion)

On 10/12/2014 1:21 PM, wrote:
John S wrote:
On 10/11/2014 12:51 PM, wrote:
John S wrote:

snip

OK, so lets analyze my results:

Conditions are free space, wire is #14 gauge but may have zero ohms
where noted. The antenna is a dipole with the source connected at the
center, F=7MHz. I'm using EZNEC with a source of 1 watt. Antenna
resonance plays no part in this. # segments = 99 unless otherwise noted.

Lambda Wire Rin Gavg(dbi) Gmax(dbi) Efficiency

0.5 zero 80 0 2.16 100%
0.5 #14 73.6 -.09 2.08 98%

0.25 zero 13.2 0 1.85 100%
0.25 #14 13.7 -.17 1.69 96%

0.125 zero 3 0 1.78 100%
0.125 #14 3.25 -.33 1.45 93%

0.05 zero .464 0 1.76 100%
0.05 #14 .556 -.78 0.98 83%

Rin is the terminal resistance only. Gave is the average gain integrated
over the pattern, Gmax is the highest gain detected.

Unless I have done something wrong, I see that a dipole that is .05
wavelengths long is within 20% of being as efficient as a half-wave
dipole. Even including wire resistance.

I invite discussion in any case.

The diameter of #14 solid wire is 0.0641"; how about a line for #8, which
is 0.1285"?


0.05 #8 0.515 -.41 1.36 91.1%


Free space, no ground loss.

So it seems that a .05 lamda dipole is only about 7% less efficient than
a full size dipole wit suitable wire...

So much for "short antennas are not efficient".

(snip extraneous input)


Yes, Jim, that is so. In fact, that was the hidden reason for the
exercise. I was hoping this would provide an example to let others know
that it is not the antenna length that is the problem as Gareth
proposed. I was hoping that others would take the investigation into
their own hands as a result.

I noted that you tried to foul me up with the unreasonable wire size.
EZNEC has a nice warning feature to take care of it.

John S wrote in :

I know of Hitch-Hiker's Guide To The Galaxy, but I have never watched.
Probably my loss. In any case, let's do the antenna thing. Yes?


I think you may need someone better at it than I am right now. I've just
been looking at pjotos of a vertical whip to consider making the junction of
fences it's mounted over into somethign better approximating thre radials of
goodish length, and it's very crude work, very basic practical salvage of an
awkward location. Apart from an FM dipole for listening, and a portanle
longwire to experiment with a Beverage antenna on open land I have access to.
This one is the likely sagging antenna, hence my interest in catenary
effects, but I'm not well placed to try modelling anything yet. I'll want to
finish the thing and get it out there just to try listening to stuff a while
first.

John S wrote in :

I know of Hitch-Hiker's Guide To The Galaxy, but I have never watched.
Probably my loss.

Not really. It's much better read, or maybe best of all, heard in the
original programs that went out on Radio 4 in the early evenings in the late
70's. Those are famous, likely easily had in various ways. I've seen the
movie, but it doesn't work for me so well. Some of it is great, but they
totally failed to get Marvin right, and that means they lost a lot of the
depth of it. I liked their Vogons though.

John S wrote in :

Ok. Putting the catenary into the simulator will take some thought from
me as well. I'll do my best, but don't wait for me. Pursue it as you
wish. (Or anybody else reading the thread)

Can somebody start another thread if you are interested?


Sweconded. I'll admit at this point welcoming an easy start, and a prepared
example of this would be a hell of an incentive for me to get into exploring
NEC too.

John S wrote:
On 10/12/2014 1:06 PM, wrote:
Lostgallifreyan wrote:
John S wrote in :

Something else might be interesting; include the effects of sag
(centenary) in a wire antenna.

Agreed. I was thinking about that possibility last night. Meaning 'catenary',
perhaps? As in 'hanging chain'? I doubt any longwire would lack this, so
modelling it would be useful.

I doubt you will see any significant difference.


Do you have any data or a simulation which supports your position?

(snip irrelevant portion)

Reread the portion you snipped as irrelevant.



--
Jim Pennino

Lostgallifreyan wrote in
:

Seconded. I'll admit at this point welcoming an easy start, and a
prepared example of this would be a hell of an incentive for me to get
into exploring NEC too.


Actually to be fair, with ,y longwire tests I'll likely be using very thin
strong stainless wire and pulling it tight enough to reduce my need to worry
about it much. STill interesting though, I've seen heavy-looking HF antennas
strung over the apex of a roof in a valley near here, and that had a
pronounced sag that may or may not have been bothersome to whoever owned it.