Help with commercial VHF mobile antenna
 

On Wed, 03 Jul 2013 18:36:44 -0700, Jeff Liebermann wrote:
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.

Gee. I sure hope the OP got "Help with commercial VHF mobile antenna".

Help with commercial VHF mobile antenna
 

On 4 Jul 2013 12:44:59 GMT, Allodoxaphobia
wrote:

Gee. I sure hope the OP got "Help with commercial VHF mobile antenna".

Well, I posted a photo of what I suspected was his "VHF" antenna,
which turned out to be an 850MHz antenna. The dimensions fit.
http://www.11junk.com/jeffl/antennas/Misc/slides/Motorola-850mhz-NMO.html
Nobody seemed to care much about answering the question. Of course,
the OP (Mr Kickin' Ass and Takin' Names) didn't bother to respond, so
I'll assume he doesn't care and has turned the problem over to the
Motorola shop, which will surely find the most expensive replacement
antenna available.


Drivel:

I wrote one of many Usenet rules in about 1995. Some applicable
quotes:

No usenet discussion can survive without topic drift after about 5
replies.

The really good postings, the ones that are illuminating, informative,
and worth keeping, usually receive no replies or comments.

The higher the authority, the bigger the mistakes.

Usenet postings are not written for the benefits of the current
reader. Rather, they are historical documents, written for the
benefit of future readers, who will then cite the incorrect
information within to perpetuate the mistakes.

Those who don't bother to trim their quotes, also don't bother to read
what's in the quotes.

One line unsubstantiated replies are usually not worth reading.


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

Help with commercial VHF mobile antenna
 

"Jeff Liebermann" wrote in message
...
On 4 Jul 2013 12:44:59 GMT, Allodoxaphobia
wrote:

Gee. I sure hope the OP got "Help with commercial VHF mobile antenna".

Well, I posted a photo of what I suspected was his "VHF" antenna,
which turned out to be an 850MHz antenna. The dimensions fit.
http://www.11junk.com/jeffl/antennas/Misc/slides/Motorola-850mhz-NMO.html

Nobody seemed to care much about answering the question. Of course,
the OP (Mr Kickin' Ass and Takin' Names) didn't bother to respond, so
I'll assume he doesn't care and has turned the problem over to the
Motorola shop, which will surely find the most expensive replacement
antenna available.


He probably got put off when many on here were siding with the Motorola shop
instead of him.

Help with commercial VHF mobile antenna
 

On Thu, 4 Jul 2013 12:48:10 -0400, "Ralph Mowery"
wrote:

He probably got put off when many on here were siding with the Motorola shop
instead of him.

It's possible. I have no way to tell. Reading between the lines in
the original posting:
"I an thinking about installing a full-length 5/8-wave whip,
but, we go into a lot of driveways with low tree limbs and I
doubt a full-length antenna would survive very long."
it seems like the antenna is on a county owned service tall van or
high truck. They probably have a service contract with the local
Motorola MSS to maintain the county owned radios. The shop is
required to use only genuine Motorola parts. If he wants to keep his
contract, he's probably stuck with using whatever the shop wants.

Incidentally, I forgot to mumble something about how to deal with tree
branch grabbing springs and coils. The problem is that a bent over
spring has large gaps into which tree branches fit nicely. When the
antenna straightens up again, it locks the branch into it's stainless
grip, and begins a tug of war. Sometimes, the grip is strong enough
to rip the antenna out of the vehicle roof.

Protecting coils are easy. Just use shrink tube over the coil.

Springs are not so easy. I tried various experiments and eventually
settled on flexible irrigation pipe or vinyl tubing. Find a size that
slips over the spring loosely. Run a ty-wrap around only the top of
the spring, not the bottom. When bent over, this sleeve will slide
upward, so make it a bit longer than the spring. I tried a sheet of
vinyl wrapped around the spring, but that tended to fall apart. Best
to use tubing.

Yes, it's ugly, but the uglier the antenna, the better it works.



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

Help with commercial VHF mobile antenna
 

On Wednesday, July 3, 2013 8:36:44 PM UTC-5, Jeff Liebermann wrote:
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?

Heck, that's what we have done for years on the lower bands.
"NVIS" On 80m, with the usual distances used for general jibber
jabber, most signals do arrive at fairly high angles.
And this was always on our minds when choosing an antenna.
But it's fairly handy that a dipole or horizontal loop at the
most used heights does shoot the bulk of the power at high angles,
with max often straight up.
For rag chew type stuff close in, a dipole is almost always preferred
over a vertical.
In my case, I always had max gain at high angles, so the only
thing left to improve was system efficiency.
Which leads me to feed with coax with no tuner used for a very
high system efficiency. Coax is slightly more lossy than ladder
line, but at 4 mhz the loss using good coax is so low you would be hard
pressed to tell the difference with the average length feed line
vs say the Cecil method using a tuned ladder line with no tuner.

Help with commercial VHF mobile antenna
 

On 6/26/2013 2:03 PM, Jeff Liebermann wrote:
On Wed, 26 Jun 2013 18:22:44 +0100, Channel Jumper
wrote:

And so Jeff speaks.

snip


The bottom line is - most people involved in communications doesn't just
start selling radios without any type of formal education.

Which apparently he hasn't had since he can't build a sentence properly.


I ran a 2-way radio shop for many years in Stanton CA. The best
salesman didn't know anything about radio. That was my job. I went
with him to meetings and filled in the techy details. Later, other
employers demonstrated the same principle. At one place, the only
technically competent person in management was the VP of engineering.
Both sales and marketing were clueless and relied on engineering to
deal with the technical details. I'm not sure how much formal
education any of these people had but they were all very effective at
selling.

Good way to do it. I am the sales engineer and the sales team knows
when they are skirting the edge of their knowledge and brings me in. As
far as I know this is how it works everywhere when there is tech involved.


Even if the only education the person received was from the Military, it
is usually based on sound practices and principals.

Nope. They are taught just enough to get it done, and often done
poorly. Unless things have changed.

If someone wants to tell me how to do something that I have been doing
for 40 years - I just walk away.

I pity your customers, since you appear to think you know it all. I'm
still learning and will until the day I die.

tom
K0TAR

Help with commercial VHF mobile antenna
 

On 7/4/2013 11:14 AM, Jeff Liebermann wrote:


One line unsubstantiated replies are usually not worth reading.


I can prove that's wrong.

tom
K0TAR

Help with commercial VHF mobile antenna
 

"tom" wrote in message
...
On 6/26/2013 2:03 PM, Jeff Liebermann wrote:
On Wed, 26 Jun 2013 18:22:44 +0100, Channel Jumper
wrote:

And so Jeff speaks.

snip


The bottom line is - most people involved in communications doesn't just
start selling radios without any type of formal education.

Which apparently he hasn't had since he can't build a sentence properly.

You mean the verb doesn't always agree with the nearest noun?!? Who'd'a
thunk it?

(That's one of my pettest of peeves, by the way.)

"Sal"
(KD6VKW)

Help with commercial VHF mobile antenna
 

On Sun, 07 Jul 2013 19:26:04 -0500, tom wrote:
On 7/4/2013 11:14 AM, Jeff Liebermann wrote:

One line unsubstantiated replies are usually not worth reading.

I can prove that's wrong.

+1

Help with commercial VHF mobile antenna
 

On 7/7/2013 9:52 PM, Sal wrote:
"tom" wrote in message
...
On 6/26/2013 2:03 PM, Jeff Liebermann wrote:
On Wed, 26 Jun 2013 18:22:44 +0100, Channel Jumper
wrote:

And so Jeff speaks.

snip


The bottom line is - most people involved in communications doesn't just
start selling radios without any type of formal education.

Which apparently he hasn't had since he can't build a sentence properly.

You mean the verb doesn't always agree with the nearest noun?!? Who'd'a
thunk it?

(That's one of my pettest of peeves, by the way.)

"Sal"
(KD6VKW)



I read scopes of work (and other corp. docs) as a significant part of my
living. In other words, I'm a proofreader. So I understand being picky
about building sentences and paragraphs and documents.

It has to make sense as a whole, and if someone can't make even a single
sentence work, they surely can't make the whole work.

tom
K0TAR