OK, let's discuss dipoles vs length
 

In message ,
writes




60 feet is good for 30M, marginal for 40M, and crap for 80M and below
for a dipole.

It is also too high for 15M and up.

For best results, a diple should be at .5 labda.

Then again, a marginal 40M dipole is better than no antenna at all.

Regardless of the theory, in real life (with real-life ground
conductivity/reflectivity, and the prevalence of nearby clutter and
obstruction), I can't help feeling in my water that 'higher generally
works better'!!



--
Ian

OK, let's discuss dipoles vs length
 

On Sun, 12 Oct 2014 12:03:43 -0500, John S
wrote:

Excellent! I like to explore and I encourage everyone to do so whether
it be with math tools or getting your hands dirty. Keep it up.

This might be useful:
http://www.amanogawa.com
Java required. Among the "Linear Antenna" animations is a common
dipole model, which shows the effects of different element lengths:
http://www.amanogawa.com/archive/DipoleAnt/DipoleAnt-2.html
I think (not sure and too lazy to RTFM) that "directivity" means gain
linear gain.

More NEC modeling tools:
http://wb0dgf.com/nec-mininec.htm

I'm partial to 4NEC2.
http://www.qsl.net/4nec2/
It comes with a huge collection of sample antennas suitable for
analysis and plagiarizing. Most of my stuff was done with 4NEC2:
http://802.11junk.com/jeffl/antennas/
(Not everything shown is my work). There are plenty of tutorials on
the web and videos on YouTube showing how to get started with 4NEC2.
https://www.youtube.com/results?search_query=4nec2
There is also a multi-threaded engine for speeding up 4NEC2. Highly
recommended:
http://users.otenet.gr/~jmsp/

If you are into reverse engineering commercial and ham antennas to see
how they work, there are a number of collections online.
http://www.hdtvprimer.com/ANTENNAS/comparing.html
http://www.hdtvprimer.com/SIMS/
http://www.qsl.net/kp4md/modeling.htm
http://www.arrl.org/antenna-modeling-files
If you want to go Googling for .NEC files, try something like this:
https://www.google.com/#q=filetype:nec+antenna
https://www.google.com/#q=filetype:ez+antenna
Some of the commercial antennas I've modeled turned out to be not very
good such as this 2.4GHz yagi:
http://802.11junk.com/jeffl/antennas/mfj1800/
(Hint: It's a 200 ohm feed, not 50 ohms).

One important "trick" is to always run an average gain test on your
design. This is a sanity check to make sure your model somewhat
resembles reality. There are at least two examples of antennas on my
web pile that do NOT pass this test and are therefore impressive
looking garbage. The results should be 1.0 for most antennas.
http://fornectoo.freeforums.org/run-average-gain-test-t355.html
http://www.arrl.org/files/file/Antenna%20Modeling%20for%20Beginners%20Supplementa l%20Files/EZNEC%20Modeling%20Tutorial%20by%20W8WWV.pdf

Please do NOT get hung up on using just one modeling program. While
the others have different interfaces and file formats, they are often
better at doing specific things. For example, I use several yagi
design programs to generate the dimensions and an NEC2 deck. I then
use EZNEC or 4NEC2 to more closely look at the result, and to add
support structures, real grounds, tapered elements, etc. While you
can buy programs that will do it all, you don't want to know the
price.

Testing antennas is also somewhat of an art. A pile of test equipment
is always nice:
http://802.11junk.com/jeffl/pics/home/slides/test-equip-mess.html
For the price of an HF radio, you can buy various "antenna analyzers"
and "network analyzers[1]" that will do a good job of emptying your
bank account. Being cheap, I just use a "return loss bridge", RF
sweep generator, and an oscilloscope. The RLB can be easily built.
These are nice:
http://www.wb.commufa.jp/ja2djh/html/e_rlb.html
Mine are ugly so no photos. There's not much to it. Just plan on
blowing out a few diodes:
http://www.qsl.net/n9zia/wireless/appendixF.html#11
http://www.qsl.net/n9zia/wireless/pics/rtrn_loss_bridge.png
What limits an RLB is that it only shows VSWR or return loss. There's
no indication if the antenna is capacitive or inductive. That's not a
real limitation as your radio could care less if the 50 ohms it sees
is resistive, capacitive, inductive, or a combination of these, as
long as the conglomeration works out to 50 ohms.

The bad news is that the RLB has to be near the antenna feed point in
order to get useful results. That usually means climbing the tower.
You can try using it at the xmitter end of the feed line, but I don't
recommend it. I recommended to one former friend how to install a
coax switch at the feed point, with one port going to an RLB. That
worked splendidly until just after he applied 500 watts to the
antenna, and fried the RLB diode. Make sure the switch has sufficient
isolation and that the RLB input is shorted when not in use.

The sweep generator is probably the most expensive part of the puzzle.
The frequency range you want to cover is always important. The HF
range is covered nicely by cheap DDS (direct digital synthesizer)
modules found all over eBay. For example:
www.ebay.com/sch/i.html?_nkw=dds+module
Add a PIC controller and output amplifier, and you have a sweeper. Or,
just buy one:
http://www.ebay.com/itm/121362216469
One problem with these is that they don't have frequency markers, but
that can be added later. For VHF and UHF, commercial equipment is
probably best. If you can find cable TV test equipment, it will
usually get you to about 400 MHz. That's three HP8620 sweepers in the
pile on the left, none of which work perfectly:
http://802.11junk.com/jeffl/pics/home/slides/BL-shop5.html

For the oscilloscope, you'll need something that is DC coupled. That
eliminates most PC sound card based software. For running an RLB, a
cheap 2.0 MHz DSO (digital storage scope) works just fine. The tiny
pocket size scopes found all over eBay will work, but the screens are
too small to see any detail. That might work if you can unload screen
captures to a computah and/or printer.

That leaves the question of why bother modeling when you can just
cut-n-try an antenna until it works? If you want to make one of
something, cut-n-try is a very good and efficient way of getting one
of anything to work. However, if you're planning on making more than
one, or publishing your design for others to copy (or steal), then
some clues as to the theoretical possibilities of the design are going
to be needed. A few years ago, I worked on a UHF antenna that was
sufficiently high in gain, and therefore narrow band, that customers
had to order it by the frequency of operation. I beat the NEC2 model
to death until everything possible was thrown into the model. U-bolts,
mounting hardware, pipes, yard arms, screws, nearby power wires, other
antennas, vehicles, buildings, etc. It was massive overkill, but very
necessary. When field tests were run on the first few installs, the
measured radiation patterns and return loss graphs were perfect. You
can't do that without a computah model.



[1] Incidentally, I've seen this problem a bit too often with MFJ
antenna analyzers and some VNA's.
http://802.11junk.com/jeffl/MFJ-269-repair/
If you own an MFJ analyzer, I suggest you stock up on matched diodes
for the inevitable rebuild.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

OK, let's discuss dipoles vs length
 

On 10/12/2014 2:59 PM, Lostgallifreyan wrote:
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.


I prefer the book to the radio program, also. A fun read - and quite
appropriate in today's world. :)

--
==================
Remove the "x" from my email address
Jerry Stuckle

==================

OK, let's discuss dipoles vs length
 

On 10/12/2014 4:29 PM, wrote:
Ralph Mowery wrote:

"Ian Jackson" wrote in message
...
In message ,
writes
Like I said, there is no difference in free space between a V and an
inverted V.

There are slight differences when close to ground.

Assuming it's a halfwave dipole, I would have thought it was better to
concentrate on getting the centre (where the current is) as high as
possible, rather than the ends. [Obviously this depends on whether you
have a suitable sky-hook available for and at the centre.] However, I
believe that there is at least one commercial loaded dipole (essentially
for portable use, I think) that has a short mast, and the ends of the V
are up in the air.



In free space I would not think there would be much of any differance.

Near the ground where most of us are is what we have to live with. In my
back yard I have enough room to string wire dipoles for 80 meters and
shorter. I don't have a way to support the antennas in the middle. I can
go up about 60 feet or so at the ends.

60 feet is good for 30M, marginal for 40M, and crap for 80M and below
for a dipole.


I'll argue with that. I got WAS on 75 meters with an inverted vee with
an apex of 50'. I was in Iowa at the time - but had a pretty good
signal across the continental U.S. late at night.

There are many things to consider, and blanket statements like yours are
pretty much worth what people pay for them.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

OK, let's discuss dipoles vs length
 

Jerry Stuckle wrote:
On 10/12/2014 2:59 PM, Lostgallifreyan wrote:
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.


I prefer the book to the radio program, also. A fun read - and quite
appropriate in today's world. :)

I have the radio shows on tape in the car, very good for a long drive. Not
a patch on the book for me, though. I've read and re-read them loads of
times since I was a teenager. And the BBC TV series from the early 80s is
superior to the 2005 film in every way.

--
Stephen Thomas Cole // Sent from my iPhone

OK, let's discuss dipoles vs length
 

Stephen Thomas Cole wrote in
:

And the BBC TV series from the early 80s is
superior to the 2005 film in every way.

I never even knew about that one! Weird.. You've both just convinced me to
try the books again though, assuming I can still cope with reading the
printed text.

OK, let's discuss dipoles vs length
 

"Ralph Mowery" wrote in
:

Looking at a chart in an old ARRL antenna handbook gives a rough
estiment of a length of 500 feet and a tension of 400 pounds a wire of
around 12 to 14 gauge will drop about 10 feet if Idid it right.


That's useful. Thanks. Did they say what metal it was? I'm guessign hard
drawn copper but if not it may be harder to adjust reckoning for something
else.

OK, let's discuss dipoles vs length
 

Jeff Liebermann wrote in
:

This might be useful:...

It it. :) I saved the whole post, to be sure I can find it fast any time. I
do have a habit of staying with one program so I'll be careful about that.
I'm already aware that NEC2 and NEC4 and MiniNEC all have things they are
particularly good at, so I'll not limit my choices the way I usually might.

OK, let's discuss dipoles vs length
 

I need to risk asking something that maybe should be obvious to me, just to
be sure I'm not starting on a very wrong assumption.. As I'm likely to only
be listening, when I see a Smith chart or other diagram indicating relative
signal transmission strength at some distance and direction, does this follow
the rule of many 'simple' transducers, in that the same chart exactly models
the sensitivity of the same antenna for reception? I'm hoping it does,
otherwise life might get complicated. :)

OK, let's discuss dipoles vs length
 

On 10/12/2014 2:14 PM, wrote:
John S wrote:
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.

What "unreasonable wire size"? #8 wire is readily available and often
used to make antenna elements, as is 1/8 th aluminum, which is only a
few thousands of an inch different.

Or are you refering to issues with segmentation and fat, short wires which I
thought I had warned you about?

Ok, wrong choice of words on my part. Sorry. Yes, I kinda thought you
were really testing me to see if I paid attention about the warning.