Supposed comparison of Mobile HF Antennas in November QST
 

Roy Lewallen wrote:
I haven't read the article, but are different mobile antennas being
compared on different vehicles, or the same vehicle? It's amazing how
many people don't realize that the vehicle is fully half the antenna,
and may in many cases play a more important role in determining overall
radiating efficiency than the supposed "antenna". So it's impossible to
draw any conclusions about mobile antennas based on comparisons done
when they're mounted on different vehicles. It's as much a test of the
vehicle's effectiveness as a radiator as it is the antenna's.

Roy Lewallen, W7EL

OK Roy you lost me.

I'll confess that I'm one of those new no code guys that concentrated on
the answer pool for three months in order to get an Extra Class license.
What I'm trying to do now is relearn enough electronics to become a
competent communicator for EMCOM purposes. You see back when I was last
in radio your transmitter would keep you warm and provide enough light
to read the manual by. Now it is thirty years since my Novice license
expired and not only are the newer radios all solid state they do most
of their signal processing digitally. By my point of view that only
makes me as yet not fully trained rather than the devils personal
representative in amateur radio.

Are you being serious when you say that much of the difference in the
mobile rigs performance may be the vehicle on which it and therefore
it's antenna are mounted? IS the body of my half ton cargo van doing a
substantial amount of the job of radiating my signal? If I mounted the
same antenna with the same mount on my Saturn should I see a difference
on a field strength meter that is the same distance to the centimeter
from the antenna over the same parking lot with the antenna over exactly
the same spot?
--
Tom Horne, W3TDH

Supposed comparison of Mobile HF Antennas in November QST
 

Roy Lewallen wrote:
Jimmie D wrote:

I once saw a demonstration that "BIG UGLY" cb antennas, these are
approximately 1/8 wl and have a huge loading coil, have more gain than
a 1/4 wl radiator. This certaily appeared to be the case when both the
antenna being test and the field strength meter's antenna were both
the "big ugly" variety and relatively close together(50'). Could this
"gain" be a result of magnetic coupling between the coils or was some
other trickery being performed.

There's no doubt this was some sort of trickery. I'd bet any amount of
money that if properly measured, the loaded 1/8 wave antenna would be
shown to have less gain due to lower efficiency. Even moderately
accurate antenna gain measurements are much more difficult to make than
most people realize, and there are many ways to be fooled. I couldn't
begin to list the all ways you could set up a demonstration like that to
get whatever result you desired.

Roy Lewallen, W7EL

Roy
For the sake of my continuing education what would I need in terms of
minimum equipment to make a meaningful comparison between antennas?
Just to keep it simple I'm working on two meters and seventy five
centimeters so far in my EMCOM prep work.
--
Tom Horne, W3TDH

Supposed comparison of Mobile HF Antennas in November QST
 

Tom Horne wrote:

OK Roy you lost me.

I'll confess that I'm one of those new no code guys that concentrated on
the answer pool for three months in order to get an Extra Class license.
What I'm trying to do now is relearn enough electronics to become a
competent communicator for EMCOM purposes. You see back when I was last
in radio your transmitter would keep you warm and provide enough light
to read the manual by. Now it is thirty years since my Novice license
expired and not only are the newer radios all solid state they do most
of their signal processing digitally. By my point of view that only
makes me as yet not fully trained rather than the devils personal
representative in amateur radio.

Fortunately, antennas still operate the same way they did back when
rectifiers glowed blue. In fact, the same as they did when you had to
poke a catwhisker around your rectifier.

Are you being serious when you say that much of the difference in the
mobile rigs performance may be the vehicle on which it and therefore
it's antenna are mounted?

Yes.

IS the body of my half ton cargo van doing a
substantial amount of the job of radiating my signal?

Yes.

If I mounted the
same antenna with the same mount on my Saturn should I see a difference
on a field strength meter that is the same distance to the centimeter
from the antenna over the same parking lot with the antenna over exactly
the same spot?

Yes.

Declaring a vehicle to be "ground" doesn't give it magic properties.
It's a conductor, just like the antenna. Exactly the same current that
flows upward on your antenna flows downward along your vehicle. The
vehicle and "antenna" comprise an asymmetrical dipole, and neither half
is inherently more or less important than the other.

Roy Lewallen, W7EL

Supposed comparison of Mobile HF Antennas in November QST
 

"Roy Lewallen" wrote in message
...
Tom Horne wrote:

OK Roy you lost me.

I'll confess that I'm one of those new no code guys that concentrated on
the answer pool for three months in order to get an Extra Class license.
What I'm trying to do now is relearn enough electronics to become a
competent communicator for EMCOM purposes. You see back when I was last
in radio your transmitter would keep you warm and provide enough light to
read the manual by. Now it is thirty years since my Novice license
expired and not only are the newer radios all solid state they do most of
their signal processing digitally. By my point of view that only makes
me as yet not fully trained rather than the devils personal
representative in amateur radio.

Fortunately, antennas still operate the same way they did back when
rectifiers glowed blue. In fact, the same as they did when you had to poke
a catwhisker around your rectifier.

Are you being serious when you say that much of the difference in the
mobile rigs performance may be the vehicle on which it and therefore it's
antenna are mounted?

Yes.

IS the body of my half ton cargo van doing a substantial amount of the
job of radiating my signal?

Yes.

If I mounted the same antenna with the same mount on my Saturn should I
see a difference on a field strength meter that is the same distance to
the centimeter from the antenna over the same parking lot with the
antenna over exactly the same spot?

Yes.

Declaring a vehicle to be "ground" doesn't give it magic properties. It's
a conductor, just like the antenna. Exactly the same current that flows
upward on your antenna flows downward along your vehicle. The vehicle and
"antenna" comprise an asymmetrical dipole, and neither half is inherently
more or less important than the other.

Roy Lewallen, W7EL

Roy is certainly right! The vehicle is the other half of the "dipole". It
is why if you stick a bugcatcher at random on a broomstick, it won't work:
SWR will be thru the roof.. Or you could hang a dipole for a given band
(your choice), then snip off the side that went to the coax shield (or vice
versa). So, yes, the vehicle is the other half of the antenna. You could
even look at it as an right-side up "L"! :)

73

Jerry
K4KWH

Supposed comparison of Mobile HF Antennas in November QST
 

Jerry wrote:

Roy is certainly right! The vehicle is the other half of the "dipole". It
is why if you stick a bugcatcher at random on a broomstick, it won't work:
SWR will be thru the roof.. Or you could hang a dipole for a given band
(your choice), then snip off the side that went to the coax shield (or vice
versa). So, yes, the vehicle is the other half of the antenna. You could
even look at it as an right-side up "L"! :)

You have to look at where the currents go. If you had a completely
symmetrical vehicle and put the antenna at the very center of the top,
the horizontal currents along the top would produce fields that almost
completely cancel, just like a bunch of radials. But when they reach the
edge and flow downward, the fields are all in the same direction and
add. So it radiates just like a very fat wire.

But if you put an antenna on the edge of the roof, then the fields from
the horizontal currents won't cancel and you'll have some radiation from
the top as well as the sides. And so forth.

Roy Lewallen, W7EL

Supposed comparison of Mobile HF Antennas in November QST
 

"Roy Lewallen" wrote in message
...

Declaring a vehicle to be "ground" doesn't give it magic properties. It's
a conductor, just like the antenna. Exactly the same current that flows
upward on your antenna flows downward along your vehicle. The vehicle and
"antenna" comprise an asymmetrical dipole, and neither half is inherently
more or less important than the other.

Roy Lewallen, W7EL


That is rather simplistic and not reality reflecting explanation.
Like saying that ground plane, vertical antenna is a dipole.
Vehicle body represents "ground plane" similar to two or more radials. The
current flows along the surface of the vehicle, just as along the more
elaborate ground plane consisting of more than say 8 radials. Cancellation
of current along the body happens, just like in opposite radials in GP.
Radiation pattern is formed between the RADIATOR (whip) and GROUND PLANE
(vehicle body).
Additional effect is that vehicle "ground plane" is capacitively coupled to
the ground and this is reflected in changes in efficiency depending on the
surroundings ground conditions (salty, wet ground, reinforced concrete
bridges, etc.)

As far as I understand, dipole refers to dual pole antenna with symmetrical
current distribution. Vertical antenna mounted on conducting body of vehicle
has current distribution in the "other pole" far from symmetrical. This can
be seen in modeling in EZNEC. Try to compare vertical whip mounted on
vehicle, with dipole that has one leg horizontal and you will see the
difference, far from "nice dipole" antenna.

So as soon as we have more than one radial, and some (horizontal)
cancellation is happening it ain't no dipole. It is monopole forming
vertical pattern against the ground plane (radials, vehicle body).

That's the way I understand it, without involving photons, Gaussss and other
farticles :-)

73 Yuri, K3BU

Supposed comparison of Mobile HF Antennas in November QST
 

Yuri Blanarovich wrote:

That is rather simplistic and not reality reflecting explanation.
Like saying that ground plane, vertical antenna is a dipole.
Vehicle body represents "ground plane" similar to two or more radials. The
current flows along the surface of the vehicle, just as along the more
elaborate ground plane consisting of more than say 8 radials. Cancellation
of current along the body happens, just like in opposite radials in GP.

It appears that you missed my explanation, so I'll try again.

The currents flowing different directions don't cancel. If they flow in
equal amounts, in phase, in opposite directions, then the fields they
create nearly cancel. And that's the case along the roof of a car if the
car and roof are symmetrical and the antenna is at the center. But it's
not the case where it flows vertically along the sides of the car.
There, the currents are in the same direction.

Radiation pattern is formed between the RADIATOR (whip) and GROUND PLANE
(vehicle body).

The radiation pattern is formed by the sum of all the fields which are
created by currents flowing on conductors. The antenna is one such
conductor. The body of the car is another.

Additional effect is that vehicle "ground plane" is capacitively coupled to
the ground and this is reflected in changes in efficiency depending on the
surroundings ground conditions (salty, wet ground, reinforced concrete
bridges, etc.)

That's true. And the coupling of the car body to ground alters the
amount of current flowing along the body of the car. This current equals
the current flowing into the antenna.

As far as I understand, dipole refers to dual pole antenna with symmetrical
current distribution.

Suit yourself. I called the system an "asymmetrical dipole". But like
"ground", putting a name on it doesn't change its properties.

Vertical antenna mounted on conducting body of vehicle
has current distribution in the "other pole" far from symmetrical. This can
be seen in modeling in EZNEC. Try to compare vertical whip mounted on
vehicle, with dipole that has one leg horizontal and you will see the
difference, far from "nice dipole" antenna.

So as soon as we have more than one radial, and some (horizontal)
cancellation is happening it ain't no dipole. It is monopole forming
vertical pattern against the ground plane (radials, vehicle body).

Certainly a whip mounted on a vehicle can be expected to have a
different pattern than a symmetrical dipole, and nothing I've written
has attempted to make a claim that it does. But it sounds like you've
grabbed onto the "asymmetrical dipole" label as a basis for argument. So
please go back over my postings but substitute "Yuri special" for
"asymmetrical dipole" and see if then you find anything I've written
which isn't correct.

That's the way I understand it, without involving photons, Gaussss and other
farticles :-)

If you say so.

Roy Lewallen, W7EL

Supposed comparison of Mobile HF Antennas in November QST
 

Roy Lewallen wrote:
Jerry wrote:

Roy is certainly right! The vehicle is the other half of the
"dipole". It is why if you stick a bugcatcher at random on a
broomstick, it won't work: SWR will be thru the roof.. Or you could
hang a dipole for a given band (your choice), then snip off the side
that went to the coax shield (or vice versa). So, yes, the vehicle is
the other half of the antenna. You could even look at it as an
right-side up "L"! :)

You have to look at where the currents go. If you had a completely
symmetrical vehicle and put the antenna at the very center of the top,
the horizontal currents along the top would produce fields that almost
completely cancel, just like a bunch of radials. But when they reach the
edge and flow downward, the fields are all in the same direction and
add. So it radiates just like a very fat wire.

But if you put an antenna on the edge of the roof, then the fields from
the horizontal currents won't cancel and you'll have some radiation from
the top as well as the sides. And so forth.

Roy Lewallen, W7EL

I think I know the answer but I need to be sure. The information you
have provided suggest that I should move the antenna from the clip on
mount at the back left corner of the van to through hole mount in the
middle cross bar of my ladder rack in order to get a more omni
directional pattern out of it right? Since the antenna is a fold over
type that I can fold to go under obstructions I think I'll still be all
right if I move it there. I have a traffic directing arrow on the back
cross bar and a full sized strobe beacon on the front cross bar. Are
there any other mobile operation pit falls that anyone would care to
point out?
--
Tom Horne, W3TDH

Supposed comparison of Mobile HF Antennas in November QST
 

"Jerry" wrote in message
...


Roy is certainly right! The vehicle is the other half of the "dipole".
It
is why if you stick a bugcatcher at random on a broomstick, it won't work:
SWR will be thru the roof.. Or you could hang a dipole for a given band
(your choice), then snip off the side that went to the coax shield (or
vice
versa). So, yes, the vehicle is the other half of the antenna. You could
even look at it as an right-side up "L"! :)

Ditto.

I worked Navy EMI for many years. The US Navy uses a variety of antennas
for HF transmissions on its ships. The hull of the ship and everything in
it conduct the HF, no matter what the antenna. Everything on a ship is
conducting HF at such levels that they're often high enough to interfere
with other equipment.

If Paul Harvey were here, he'd say, " ... and now you know the rest of the
antenna."

Supposed comparison of Mobile HF Antennas in November QST
 

Tom Horne wrote:

Roy
For the sake of my continuing education what would I need in terms of
minimum equipment to make a meaningful comparison between antennas? Just
to keep it simple I'm working on two meters and seventy five centimeters
so far in my EMCOM prep work.

That's a tall order. What characteristics of the antennas are meaningful
to you? Gain in some particular direction and elevation angle, minimum
gain at any azimuth at some elevation angle, pattern circularity,
mechanical ruggedness, corrosion resistance, portability, size, weight,
SWR at some specific frequency or over some particular band, efficiency?
Does it have to have some characteristic(s) when mounted on some
particular vehicle? Some point on some vehicle? Some class of vehicles?

What sort of accuracy would make the results meaningful?

Roy Lewallen, W7EL