Two Meter FM Antenna Question
 

Every reference and factory specification for
standard grounplane and vertical dipole antennas I've seen indicates
the
standard vertical dipole has a horizontal gain of 3 dBi, and the
groundplane 2.1 dBi.



Those are pretty curious references then. Why would they dwell on the
HORIZONTAL gains of VERTICAL antennas? There must be something left
unsaid in what you are trying to express because cross polarization
would drive down sensitivities by 20 to 30 dB.

For another matter, those values you quote bear very little
resemblence to typical 2M FM operation, unless it is from the Space
Shuttle. Height above ground variations in gain easily washes over
any differences you might perceive. I can see a variation of 2dB in
just raising a groundplane from 40" off the turf to 120".


Richard,

You and I seem to be talking different languages !! :^)

My reference to horizontal gain is gain measured in the horizontal
plane...... that is, measurements are taken broadside to the vertical
antenna elements, in this case, both the vertical dipole and a
groundplane.

Gain measurements taken in any other plane in any other plane than
horizontal tend to be rather useless since most VHF mobile communications
takes place horizontally..... even distant repeaters tend to be close to
the horizon.

Cross polarization is not an issue in VHF operations since all
commercial and amateur FM operations I'm familiar with use vertical
polarization.

As far as height variations having effect on gain.... you are
talking about path gain, or system gain. I am speaking specifically of
antenna gain, which I believed to be the question of the original poster.

And as I have already pointed out, most factory specifications
for vertical dipoles and groundplane antennas are as I already listed.

Ed K7AAT

Two Meter FM Antenna Question
 

You should always run an average gain test when you have an unusually
high or low field strength. Using EZNEC, the average gain shows as
1.227, or 0.89 dB, and this same average gain should be reported by
NEC-2. That means that the actual gain is 0.89 dB less than what NEC-2
is reporting, or just about 2.0 dBi. If you don't understand what this
test is, consult the NEC-2 manual. EZNEC users will find it indexed in
the EZNEC manual.

I suggest you also do an average gain check on your 3 dBi vertical.

Roy Lewallen, W7EL

John E. Davis wrote:
On 21 Aug 2006 17:50:21 GMT, Ed
Its all a matter of reference. I was thinking in terms of dBi.... a
vertical has 3dBi gain, a ground plane, 2 dBi.

Here is a ground plane with a free-space gain greater than 2.9 dBi and
an SWR less than 1.2 at its design frequency (as given by NEC2):

CM Groundplane antenna for MURS (151.8 Mhz)
CE
GW 1 19 0 0 0 0 0 0.444243 0.000813863
GW 2 21 0 0 0 0 0.163546 -0.4506 0.000813863
GW 3 21 0 0 0 0 -0.163546 -0.4506 0.000813863
GW 4 21 0 0 0 -0.163546 0 -0.4506 0.000813863
GW 5 21 0 0 0 0.163546 0 -0.4506 0.000813863
GE 0
FR 0 31 0 0 145 0.33
EX 0 1 1 0 1
RP 0 31 73 1001 0, 0, 3, 5, 10000, 0
EN


It uses 14 AWG wire and consists of a 17-1/2 inch vertical, and 4
18-7/8 inch radials symmetrically placed at about 20 degrees with
respect to the vertical axis:

|
|
| A A = 17-1/2 in
| B = 18-7/8 in
| T = 2*19.95 degrees
/ \ C = 12-7/8 in
/ T \ B
/ \ (only 2 radials shown)
/ \
-- C --

--John

Two Meter FM Antenna Question
 

On 22 Aug 2006 00:21:47 GMT, Ed
wrote:



And as I have already pointed out, most factory specifications
for vertical dipoles and groundplane antennas are as I already listed.

Ed K7AAT

Maybe I'm missing something, who manufactures vertical vhf dipoles?

bob
k5qwg

Two Meter FM Antenna Question
 

Maybe I'm missing something, who manufactures vertical vhf dipoles?

bob
k5qwg




Cushcraft used to sell one, amongst others. Most commercial
dipoles now are folded design, and designed for side mount on a tower or
mast, so gain figures tend to include the effects of the mast... although
they can be mounted on top of a tower or mast, too. Other than the
certain physical advantages inherent in folded dipole design, the
performance remains about the same as a standard halfwave dipole.

A quick check showed Andrew and Celwave (RFS Celwave) with current
products along these lines. Couldn't find a decent site for Cushcraft,
and didn't spend time looking for other vendors.

http://www.andrew.com/products/anten...a/DB220-B.aspx

http://shop.talleycom.com/store/product.jsp?
pdtl=_root&pdtl_pn=TELANT150D


Ed

Two Meter FM Antenna Question
 

Bob Miller wrote:
On 22 Aug 2006 00:21:47 GMT, Ed
wrote:



And as I have already pointed out, most factory specifications
for vertical dipoles and groundplane antennas are as I already listed.

Ed K7AAT


Maybe I'm missing something, who manufactures vertical vhf dipoles?

bob
k5qwg



I don't know about vertical dipoles for ham radio but many manufacturers
make them for marine applications. The vhf marine band antennas for
fiberglass boats are center fed vertical dipoles.

Dave WD9BDZ

Two Meter FM Antenna Question
 

"Bob Miller" wrote in message
...

snip


Maybe I'm missing something, who manufactures vertical vhf dipoles?

bob
k5qwg


US Navy has two, at least, NT-66095 for the VHF aircraft band and a
shortened version, NT-66095MOD, for the 160 MHz Marine band. Also, do a
Google search on "dipole sleeve" and "dipole cage" to see a few others.
Military also uses some biconical dipoles for receive only. They look like
two funnels, connected together at the skinny end and are all mounted
vertically for omni coverage, IIRC.

Would it be cheating to call the J-pole a vertical dipole? It is a
free-space half-wave radiator, albeit end-fed.

Two Meter FM Antenna Question
 

Sal M. Onella wrote:
"Bob Miller" wrote in message
...

snip

Maybe I'm missing something, who manufactures vertical vhf dipoles?

bob
k5qwg


US Navy has two, at least, NT-66095 for the VHF aircraft band and a
shortened version, NT-66095MOD, for the 160 MHz Marine band. Also, do a
Google search on "dipole sleeve" and "dipole cage" to see a few others.
Military also uses some biconical dipoles for receive only. They look like
two funnels, connected together at the skinny end and are all mounted
vertically for omni coverage, IIRC.

Would it be cheating to call the J-pole a vertical dipole? It is a
free-space half-wave radiator, albeit end-fed.



Actually it looks like a OCF dipole antenna to me.

Dave WD9BDZ

Two Meter FM Antenna Question
 

On Mon, 21 Aug 2006 18:31:54 -0700, Roy Lewallen
wrote:
You should always run an average gain test when you have an unusually
high or low field strength. Using EZNEC, the average gain shows as
1.227, or 0.89 dB, and this same average gain should be reported by
NEC-2. That means that the actual gain is 0.89 dB less than what NEC-2
is reporting, or just about 2.0 dBi. If you don't understand what this
test is, consult the NEC-2 manual. EZNEC users will find it indexed in
the EZNEC manual.

You are correct--- an antenna cannot radiate more energy than was
input into it. Increasing the number of segments did not help the
accuracy of the model either. It appears that the angles are too
acute (40 degrees) for NEC-2 to model accurately. Changing the
geometry to avoid the smaller angles also produced an antenna with a
gain of 2 dBi as you suggested.

Perhaps you can advise me regarding the numerical stability of the
following omni-directional, which NEC indicates has a gain of 4 and a
VSWR1.2 when fed with a 50ohm feedline at 151.75 Mhz.

The geometry consists of a 54.5 inch vertical with 4 35.625 inch
radials that are bent upward by about 11 degrees. The vertical is
made from 14 AWG wire, while the radials are 1/8 inch brazing rod.

CM High Gain Omni for MURS
CE
GW 1 29 0 0 0 0 0 1.38375 0.000813863
GW 2 3 0 0 0 0.0508 0 0 0.0015875
GW 3 19 0.0508 0 0 0.938496 0 0.172104 0.0015875
GW 4 3 0 0 0 3.1105e-18 0.0508 0 0.0015875
GW 5 19 3.1105e-18 0.0508 0 5.74644e-17 0.938496 0.172104 0.0015875
GW 6 3 0 0 0 -0.0508 6.221e-18 0 0.0015875
GW 7 19 -0.0508 6.221e-18 0 -0.938496 1.14929e-16 0.172104 0.0015875
GW 8 3 0 0 0 -9.3315e-18 -0.0508 0 0.0015875
GW 9 19 -9.3315e-18 -0.0508 0 -1.72393e-16 -0.938496 0.172104 0.0015875
GE 0
FR 0 41 0 0 145 0.25
EX 0 1 1 0 1
RP 0 61 73 1001 0 0 3 5 10000, 0
GN -1 0 0 0 0.0 0.0 0.0 0.0 0.0 0.0
EN

With segment sizes of 0.05 and 0.025 lambda, the average power gain is
very close to 1. Changing the segment size to 0.0125, drops the
average power gain to 0.93, which indicates numerical instability.

Should I believe this model?

Thanks again,
--John

Two Meter FM Antenna Question
 

Let's not confuse "efficiency" with "gain". Efficiency asks, "Of the power
that is incident at the feed point of the antenna, how much of that power is
radiated into free space and how much is wasted as reflected or consumed in
losses (matching or the elements themselves)?"

Gain asks, "For a receiver far distant (in terms of wavelengths) from the
antenna, which antenna produces a higher signal strength?"

In the latter, which is what I presume you meant, we have to have a
reference of some sort ... we have a fictitious impossible antenna called
"isotropic", which says that all power is radiated from a point source that
is infinitely small and infinitely efficient ... that is, all the power
incident on the point is radiated equally in all spherical directions ... a
radiating molecular seed at the center of an orange the size of Yankee
stadium.

If you measure a perfect dipole with respect to this isotropic source, you
find a "gain" perpendicular to the dipole elements of 2.14 dB. Where did
this "gain" come from, since power can not be created by a passive antenna?
If you look at the radiation pattern of a dipole, this apparent increase in
power was caused by a deep hole in the pattern off the ends of the dipole.
THe dipole, in essence, squeezed the top and the bottom to let the sides
bulge out. Think of a donut dropped over the elements and sitting at the
feed point of the dipole.

For the ground plane, think of that same donut cut in half through the fat
part of the donut. Now since our "power" is really the volume of the donut,
if you cut it in half, you are going to have to start out with a fatter
donut if you are going to wind up with the same volume. Now drop that fat
donut over the radiating element and let it come to rest on the center of
the ground plane. The dipole radiated its energy so that half of it was
"up" and half of it was "down". If "down" into the ground isn't what you
wanted, then the ground plane, which radiates all of its energy "up" at some
angle to the horizon, has more gain. By a clever bending of the ground
plane wires down at some angle to the horizon, you can move that donut
pattern down to where it is nearly horizontal. Thus, for a person at some
far distance, a properly constructed ground plane will appear to have a
stronger signal relative to a dipole.

Jim



"N3" wrote in message
oups.com...
Which one of the the two is more efficient as a radiator & why?

1/2 wave vertical fed in the center with coax or one vertical 1/4 wave
with four 1/4 wave radials also fed in the center with coax?

Two Meter FM Antenna Question
 

"N3" wrote in message
oups.com...
Which one of the the two is more efficient as a radiator & why?

1/2 wave vertical fed in the center with coax or one vertical 1/4
wave
with four 1/4 wave radials also fed in the center with coax?

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

They are both equally efficient. There's no reason why they should be
otherwise.

Hint : Try not to confuse efficiency with gain.