VHF Ground plane radials
 

When making the standard VHF-UHF ground plane antenna as illustrated in
all the books, with four ground radials sloped down at a 45 degree
angle, are those radials a tuned length? How much difference would it
make if they were some random length considerably longer than specified?
(as, for example, using the antenna on a different band by merely
changing the center vertical element.)

VHF Ground plane radials
 

The antenna you're describing is a dipole. Half of the dipole is the
upper vertical wire. The lower half is the radials. Changing the length
of one half has the same general effect as changing the other half.

Roy Lewallen, W7EL

David Harmon wrote:
When making the standard VHF-UHF ground plane antenna as illustrated in
all the books, with four ground radials sloped down at a 45 degree
angle, are those radials a tuned length? How much difference would it
make if they were some random length considerably longer than specified?
(as, for example, using the antenna on a different band by merely
changing the center vertical element.)

VHF Ground plane radials
 

If the ground radials go from 45 degree sloping downward to straight
horizontal, at what point does it become a ground plane and not a dipole?

Roy Lewallen wrote:
The antenna you're describing is a dipole. Half of the dipole is the
upper vertical wire. The lower half is the radials. Changing the length
of one half has the same general effect as changing the other half.

Roy Lewallen, W7EL

David Harmon wrote:
When making the standard VHF-UHF ground plane antenna as illustrated in
all the books, with four ground radials sloped down at a 45 degree
angle, are those radials a tuned length? How much difference would it
make if they were some random length considerably longer than specified?
(as, for example, using the antenna on a different band by merely
changing the center vertical element.)

VHF Ground plane radials
 

On May 24, 1:14 am, Rick wrote:
If the ground radials go from 45 degree sloping downward to straight
horizontal, at what point does it become a ground plane and not a dipole?



Roy Lewallen wrote:
The antenna you're describing is a dipole. Half of the dipole is the
upper vertical wire. The lower half is the radials. Changing the length
of one half has the same general effect as changing the other half.

Roy Lewallen, W7EL

David Harmon wrote:
When making the standard VHF-UHF ground plane antenna as illustrated in
all the books, with four ground radials sloped down at a 45 degree
angle, are those radials a tuned length? How much difference would it
make if they were some random length considerably longer than specified?
(as, for example, using the antenna on a different band by merely
changing the center vertical element.)- Hide quoted text -

- Show quoted text -

The antenna is not a dipole unless it has only one "radial".
A ground plane is just that...several radials sloping and creating the
surface seen by the vertical element.

VHF Ground plane radials
 

On Thu, 24 May 2007 04:14:00 GMT, Rick wrote:

If the ground radials go from 45 degree sloping downward to straight
horizontal, at what point does it become a ground plane and not a dipole?

Hi Rick,

It becomes a "ground plane" (as you think of it) only when you plant
the radials (fully horizontal) into the ground - otherwise, with
elevation, it is an elaborate vertical dipole.

73's
Richard Clark, KB7QHC

VHF Ground plane radials
 

It's unfortunate that so many people feel the need to divide antennas
like so many other things into binary categories. When the radials are
horizontal, they radiate very little. As they're bent downward, they
radiate more and more. So the transition, if you must have one, occurs
wherever you want to consider the radiation to be "significant".

But even a ground plane with horizontal radials behaves like a dipole in
some respects, like current distribution. So if you make the radials
longer, it has an effect on the current distribution that's similar to
making the top longer, but in the opposite direction. And the impedance
change caused by lengthening the radials is similar to that of
lengthening the top. So in some ways it's still a dipole.

The point I was hinting at is that calling ground plane radials "ground"
doesn't impart magical properties to them. They are conductors with
current flowing on them, so they create fields like any other conductors
with currents. And the total current flowing in the radials is equal to
the current flowing in the top wire.

Roy Lewallen, W7EL

Rick wrote:
If the ground radials go from 45 degree sloping downward to straight
horizontal, at what point does it become a ground plane and not a dipole?

Roy Lewallen wrote:
The antenna you're describing is a dipole. Half of the dipole is the
upper vertical wire. The lower half is the radials. Changing the
length of one half has the same general effect as changing the other
half.

Roy Lewallen, W7EL

David Harmon wrote:
When making the standard VHF-UHF ground plane antenna as illustrated in
all the books, with four ground radials sloped down at a 45 degree
angle, are those radials a tuned length? How much difference would it
make if they were some random length considerably longer than specified?
(as, for example, using the antenna on a different band by merely
changing the center vertical element.)

VHF Ground plane radials
 

Roy Lewallen wrote in
:

....
The point I was hinting at is that calling ground plane radials
"ground" doesn't impart magical properties to them. They are
conductors with current flowing on them, so they create fields like
any other conductors with currents. And the total current flowing in
the radials is equal to the current flowing in the top wire.

.... if you assume that no current flows on the feedline / mast, and it
seems that doesn't happen by accident.

Owen

VHF Ground plane radials
 

David Harmon wrote:
When making the standard VHF-UHF ground plane antenna as illustrated in
all the books, with four ground radials sloped down at a 45 degree
angle, are those radials a tuned length? How much difference would it
make if they were some random length considerably longer than specified?
(as, for example, using the antenna on a different band by merely
changing the center vertical element.)


So, these guys are telling you that the antenna you describe is a
dipole? Funny, you call it a ground plane antenna (gpa) and I can draw
a mental picture of exactly what you are attempting to build. If you
had called it a dipole, I'd have expected the common half wave wire fed
in the middle.

You get my drift, nice to have two different names for the two vastly
mechanically-differing constructions of metal and insulators.

By the way, have you ever heard a yagi called a "Multiple Dipole Antenna
Working in Tandem?" GRIN!

As to your question, cut the radials ~5% longer than the vertical
element, droop them at a angle which give you a good match to your 50
ohm coax (or ~45 degrees.)

Better yet, consider a half wave end fed antenna which requires no
counterpoise (radials, check out the radiation pattern of the halfwave
endfed):
http://www.70mhz.org/halfwav.htm
Simply change dimensions to fit your freq(s), much easier to hang a
single vertical wire in the air than one vertical and four
horizontal--no matter what the freq.

Best regards,
JS