On Thu, 27 Jun 2013 16:11:50 -0700, Jeff Liebermann
wrote:

I'm stuck at home today with a foot problem. So, I get to sit at the
computer instead of the workbench. I'll throw together a web page
showing that cutting the antenna short does NOT reduce it's gain and
efficiency very much (but does mangle the pattern and VSWR). Stay
tuned.

5 hours (minus dinner) later and I'm dead tired. What started as a
simple little demonstration turned into a time burning nightmare.
Here's where I stopped:
http://www.11junk.com/jeffl/antennas/Monopole/index.html
The various sub-directories are NEC2 models for various length
monopole antennas over a perfect ground plane. That a rough
approximation of what one would expect to see on the roof of a car
with a large metal roof at VHF/UHF frequencies. It's not quite
correct, but close enough for this exercise.

The directories are named after the length of the monopole antenna.
For example:
monopole_0_625
is a 0.625 or 5/8th wavelength antenna. The underscores were used
because Windoze XP detests more than one period in a filename.

The NEC deck is really simple.

CM Monopole antenna over perfect ground.
CM by Jeff Liebermann AE6KS 06/25/2013
CE
SY LENGTH = 0.625 'Length in wavelengths
GW 1 21 0 0 0 0 0 LENGTH 0.001
GE 1
GN 1
EK
EX 0 1 1 0 1 0
FR 0 0 0 0 299.8 0
EN

The only value that changes for each antenna is the label:
LENGTH = X.XXX
The 0.001 is 0.001 wavelengths for a wavelength = 1 meter, which is a
2mm diameter monopole antenna. The 299.8 MHz frequency is a
convenient trick to make 1 wavelength equal to 1 meter, making all the
dimension appear in wavelengths. That allow this antenna to be easily
scaled to any frequency.

If you feel ambitious, download and install 4NEC2 from:
http://www.qsl.net/4nec2/
and try it. If you're really into big models, I suggest you also get
the multi-core/processor NEC2 engine from:
http://users.otenet.gr/~jmsp/
which really speeds things up.

So much for the background stuff...

Start with the 1/4 wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/index.html
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/slides/monopole_0_250.html
Note that the gain is 5.19dBi. At this point, I usually get an
outrage from everyone who knows that a dipole is 2.15dBi and that this
monopole can't possibly have more. Well, we have a perfectly
reflective ground under this antenna, that reflects 100.0% of
everything that hits it, effectively doubling the gain.
2.15dBi + 3.01dB doubling = 5.16dBi
You'll see the extra 3dB gain throughout the various pages.

The common misconception is that shorter antennas have less gain. Yes,
they do, but it's not really proportional to the length. For example,
the 1/4 wave monopole may have 5.19dBi gain,
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/slides/monopole_0_250.html
but the 1/8th wave monopole still has 4.86dbi gain
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html
and the 1/20th wave monopole still has 4.79dBi gain.

Going the other direction with longer monopole antennas, the full wave
monopole at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_1_000/slides/monopole_1_000.html
has 7.06dBi gain or less than 2dB more than a 1/4 monopole. One might
expect that having 4 times as much wire as the 1/4 wave monopole would
produce a 6dB gain increase, but that's not how it works.

I did some tweaking and arranged to produce the antenna impedance in
polar form. For example, the 1/4 wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/slides/monopole_0_250.html
has an impedance of 48.7 ohms with a phase angle of 30.2 degrees.
Close enough to 50 ohms.

However, as we get into even multiples of 1/4 wavelength, the
impedances become very high. For example, the infamous 1/2 wave
monopole shows 934 ohms:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_500/slides/monopole_0_500.html
which is not going to be easy to match.

On the short end of the scale, the 1/8th wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html
shows 254 ohms, which will work with a 2:1 turns ratio transformer.

If you look at the antennas that are odd multiples of 1/4 wavelength,
you'll notice that their impedances are tolerably close to 50 ohms.
For example, the 1.25 wavelength antenna is 72.9 ohms, which will
probably work without any matching xformer.
http://www.11junk.com/jeffl/antennas/Monopole/monopole_1_250/slides/monopole_1_250.html

If you look at the patterns at:
http://www.11junk.com/jeffl/antennas/Monopole/index.html
you'll see some interesting things. The pattern for the 1/2 wave
monopole and shorter are all very similar. The gain is also fairly
constant. I can't say the same for the impedance, which varies
radically and the takeoff angle, which keeps creeping upward as the
antenna gets longer. As the antenna gets really long, such as this 5
wavelength monopole monster:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_5_000/slides/pattern.html
the major lobes are almost straight up, which might be useful for
talking to satellites but not terrestrial repeaters. Note that the
gain has increased to 10.7dBi or 5.5dB more than the 1/4 wave
monopole.

Lots more can be extracted from the simulations. I'll clean up the
mess, contrive a web page, make it pretty, but not tonite.








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

On 6/27/2013 11:49 PM, Jeff Liebermann wrote:

On the short end of the scale, the 1/8th wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html
shows 254 ohms, which will work with a 2:1 turns ratio transformer.

I don't think a transformer is a significant help. Without the
transformer the SWR is about 158:1. With the transformer, the SWR is
still up to about 61:1. That will probably kick in the SWR protection of
the transmitter.

John - KD5YI

On Sun, 30 Jun 2013 07:24:34 -0500, John S
wrote:

On 6/27/2013 11:49 PM, Jeff Liebermann wrote:

On the short end of the scale, the 1/8th wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html
shows 254 ohms, which will work with a 2:1 turns ratio transformer.

I don't think a transformer is a significant help. Without the
transformer the SWR is about 158:1. With the transformer, the SWR is
still up to about 61:1. That will probably kick in the SWR protection of
the transmitter.

John - KD5YI

Nope. A 2:1 turns ratio tranformer will provide a 4:1 impedance
ratio, not a 2:1 impedance ratio.

The required transformer ratio would be:
(254 / 50)^0.5 = sqrt(5) = 2.3
A 2:1 turns ratio xformer should be close enough.

Another way is to take the 2:1 turns ratio transformer, which has a
4:1 impedance ratio, and divide the antenna impedance by the impedance
ratio:
254 / 4 = 63.5 ohms.
Not exactly 50 ohms, but close enough.






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

On Sun, 30 Jun 2013 10:58:08 -0700, Jeff Liebermann
wrote:

On Sun, 30 Jun 2013 07:24:34 -0500, John S
wrote:

On 6/27/2013 11:49 PM, Jeff Liebermann wrote:

On the short end of the scale, the 1/8th wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html
shows 254 ohms, which will work with a 2:1 turns ratio transformer.

I don't think a transformer is a significant help. Without the
transformer the SWR is about 158:1. With the transformer, the SWR is
still up to about 61:1. That will probably kick in the SWR protection of
the transmitter.

John - KD5YI

Nope. A 2:1 turns ratio tranformer will provide a 4:1 impedance
ratio, not a 2:1 impedance ratio.

The required transformer ratio would be:
(254 / 50)^0.5 = sqrt(5) = 2.3
A 2:1 turns ratio xformer should be close enough.

Another way is to take the 2:1 turns ratio transformer, which has a
4:1 impedance ratio, and divide the antenna impedance by the impedance
ratio:
254 / 4 = 63.5 ohms.
Not exactly 50 ohms, but close enough.

Oops. My mistake. I couldn't recall if a 2:1 transformer referred to
the turns ratio or the impedance ratio. I've seen it done both ways
in other industries and transformer applications. I usually qualify
the label with either turns or impedance ratio but forgot this time.
However, skimming the available literature with Google, I find that
the common usage for RF xformers is the impedance ratio. Therefore,
your comments are correct and I should have specified a 4:1
transformer. Sorry(tm).



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

On 6/30/2013 1:05 PM, Jeff Liebermann wrote:
On Sun, 30 Jun 2013 10:58:08 -0700, Jeff Liebermann
wrote:

On Sun, 30 Jun 2013 07:24:34 -0500, John S
wrote:

On 6/27/2013 11:49 PM, Jeff Liebermann wrote:

On the short end of the scale, the 1/8th wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html
shows 254 ohms, which will work with a 2:1 turns ratio transformer.

I don't think a transformer is a significant help. Without the
transformer the SWR is about 158:1. With the transformer, the SWR is
still up to about 61:1. That will probably kick in the SWR protection of
the transmitter.

John - KD5YI

Nope. A 2:1 turns ratio tranformer will provide a 4:1 impedance
ratio, not a 2:1 impedance ratio.

The required transformer ratio would be:
(254 / 50)^0.5 = sqrt(5) = 2.3
A 2:1 turns ratio xformer should be close enough.

Another way is to take the 2:1 turns ratio transformer, which has a
4:1 impedance ratio, and divide the antenna impedance by the impedance
ratio:
254 / 4 = 63.5 ohms.
Not exactly 50 ohms, but close enough.

Oops. My mistake. I couldn't recall if a 2:1 transformer referred to
the turns ratio or the impedance ratio. I've seen it done both ways
in other industries and transformer applications. I usually qualify
the label with either turns or impedance ratio but forgot this time.
However, skimming the available literature with Google, I find that
the common usage for RF xformers is the impedance ratio. Therefore,
your comments are correct and I should have specified a 4:1
transformer. Sorry(tm).

No problem and no reason to apologize.

For the sake of those who read this forum, I will provide my analysis
upon request.

John - KD5YI

The ladder line portion of the G5RV is the matching network.

You cannot use a tuner with a tuner.

If the matching network is the ladder line and you connect a tuner to it - yes you can trick the transceiver into believing that is is seeing a 50 ohm matched load - but all you are going to create is heat.

On the other side of the coin, I hear all the time - I can work everything that I can hear - with my G5RV - the problem is - what can you hear?

Unless you have a real 80 meter dipole and you compare them side by side - within one hour of each other, at the same height and in the same neighborhood - you cannot compare the two.

In the end - you will realize that the efficiency is so low - you are not hearing much - just the strongest of signals - when the band is open, and not much of anything when the bands are no cooperating.

The thing that tricks people into thinking that they are doing something is the fact that they see 100 watts into the meter and they think that they are modulating all 100 watts - when in fact a single side splatter signal is only fully modulated part of the time - most of the time - we aren't really using more then maybe 15 or 20 watts out of 100.

Only the digital modes and CW - which is the original digital modes - dots and dah's - is 100% fully modulated.

That is the reason why we turn down the power when we work digital modes.
Most transceivers do not have a 100% duty cycle - hence if you operate at 100 watts for very long - your transceiver will not take it!

On Thu, 4 Jul 2013 00:33:18 +0100, Channel Jumper
wrote:

You cannot use a tuner with a tuner.

Nope. I've done that for fun. I just happen to have two identical
MFJ tuners available and thought it might be amusing to put them back
to back and measure the losses at the 50 ohm output. One tuner was
set to be capacitive, while the other was matched to have the
conjugate inductive reactance. It worked nicely until I tried 80
meters, where I heard some internal arcing. Measured losses were
fairly high on 40 and 75 meters.

If the matching network is the ladder line and you connect a tuner to it
- yes you can trick the transceiver into believing that is is seeing a
50 ohm matched load - but all you are going to create is heat.

Baloney. The losses come from the limited Q and high resistive losses
of the inductors used in the antenna tuner. That's why really good
antenna tuners use big fat silver plated coils. Try it yourself with
this Java app:
http://www.rsq-info.net/PSK-modelling.html
You'll start to see substantial losses on 80 meters with the default
values. The example uses Q=100 for the inductor, which might be a bit
optimistic for 80 meters. (I haven't done a tuner in 30 years so I
forget the typical Q values). If you plug in real values extracted
from your favorite MFJ antenna tuner, you'll see losses at higher
frequencies.

On the other side of the coin, I hear all the time - I can work
everything that I can hear - with my G5RV - the problem is - what can
you hear?

Unless you have a real 80 meter dipole and you compare them side by side
- within one hour of each other, at the same height and in the same
neighborhood - you cannot compare the two.

In the end - you will realize that the efficiency is so low - you are
not hearing much - just the strongest of signals - when the band is
open, and not much of anything when the bands are no cooperating.

Sigh. In the 1970's, I did some work with diversity reception on HF.
In order for diversity to work, the reception between the two antennas
needed to be different presumably via a different skywave path. The
tests were on WWV at 2.5, 5.0, 10.0, and 15.0Mhz with a simple dipole
and balun tuned to 5.0Mhz. We started with the antennas on opposite
sides of the parking lot. The signal levels tracked each other. I
ran 1000ft of RG-58c/u down the roadway and the signal still tracked.
I ran another 1000ft down the roadway in the opposite direction, and
the signals still tracked. I moved one of the receivers about 10,000
ft away and ran twisted pair audio back to the factory. Finally, with
11,000ft of separation, I was able to see frequency selective fading
at HF frequencies suitable for diversity reception. (Incidentally,
this was adjacent to SJO airport, which added a political layer to
such testing).

The real problem with comparing antennas closely located is that they
interact with each other. Ideally, I would want to see 2-3
wavelengths separation between antennas to prevent interaction. Well,
at 80 meters, that's 500 to 750 ft separation, which is difficult to
achieve.

For added amusement and confusion, there's the commonly ignored
problem of takeoff angle. The usual drawings in the books show a
signal bouncing between the ground and the ionosphere several time
with the angle of incidence equal to the angle of reflection. We'll
it doesn't quite work like that. There was an article in QST last
year demonstrating that the signal comes from directly overhead. While
DX'er try to optimize the takeoff angle to match the equal angles of
incidence and reflection, perhaps it would more interesting to try
maximizing the gain straight up? I'll see if I can find the issue and
article.

How the G5RV fits into the picture is beyond my limited imagination.

The thing that tricks people into thinking that they are doing something
is the fact that they see 100 watts into the meter and they think that
they are modulating all 100 watts - when in fact a single side splatter
signal is only fully modulated part of the time - most of the time - we
aren't really using more then maybe 15 or 20 watts out of 100.

Well, you can set the % modulation to 100% and get 100% modulation.
The problem is that it can easily splatter as you describe. 25% of CW
power is the recommended maximum.

Note that none of this diversion has anything to do with antennas.

Only the digital modes and CW - which is the original digital modes -
dots and dah's - is 100% fully modulated.

Wrong. Percent modulation is the radio of the peak-to-peak voltage at
the waveform peaks, divided into the peak-to-peak voltage in the
modulation troughs, as shown on an oscilloscope. 100% is very common
and easily obtained. Please look at the RF on a scope and see for
yourself.
http://electriciantraining.tpub.com/14193/css/14193_146.htm

That is the reason why we turn down the power when we work digital
modes.

Nope. The reason we turn down the percent modulation is to reduce
splatter, not because the transmitter is somehow inherently unable to
produce 100% modulation.

Most transceivers do not have a 100% duty cycle - hence if you operate
at 100 watts for very long - your transceiver will not take it!

Wrong again. The reason for the low percentage of modulation for most
digital modes is to keep the occupied bandwidth fairly reasonable. As
you approach 100% modulation, the signal starts to become wide and
begins to splatter. Beyond 100%, it's really wide and ugly. Here's
the math for PSK31:
http://www.rsq-info.net/PSK-modelling.html
Compare the occupied bandwidth and spurious junk at 25% modulation
(Fig 3) with the others showing various anomalies.



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