Terry Bartholomew NQ4Y wrote:
I am on a fact finding mission, gathering information about
the pros and cons of a particular antenna design/configuration.
This antenna will be used on a VHF repeater (Monteagle, TN.)
We need near 360 degree coverage. The repeater is at 2300
feet HAMSL if downtilt should be a consideration.
I have narrowed the choices down to either an 8 or a 16 element
dipole array.
1) Would a folded dipole have more gain than a "gamma matched"
dipole? I was told that a folded dipole has 1.5dB more
gain than a standard dipole.
I don't know if it is a full 1.5 db, but due to matching efficiencies, it is
better. One thing that is definitely better with a folded dipole is
operating bandwidth, a Gamma match fed dipole is limited in bandwidth as
compared to a true folded dipole.
2) What is the best way to orient the antennas on the tower
(Rhon 25G?)
a) put them all on the same side of the tower and point
them toward the tower...
See below for an explanation.
b) put them in two groups of 8, back to back...
c) start at the top and mount them down the tower, turning
each 90 degrees from the previous...
d) put them in two groups of 8, first group of 8 over the
second group of 8, point the lower group of 8 180 degrees
of the first 8?
3) Has anyone had any experience with the Maxrad 4 element dipole
array? Or would it perform like the old cushcraft 4 element
dipole?
All comments welcome!
-Terry NQ4Y
Since you said you need "near 360 degree coverage", I'll suppose you could
stand a slight null in one direction.
In exposed dipole arrays the mounting pole has an effect on coverage. In a 4
bay array with all elements in the same direction there is a null in the
direction of the pole. In an effect to counter the null... one can turn the
elements in all four directions to try to overcome this. As you know
(because of the laws of constructive and destructive interference's)
"maximum gain" can only be achieved by having the wave frontal hit all the
elements at the same time. In low band this theory works well, but on 2
meters there starts to be a compromise in available gain when the elements
are positioned around the pole due to the wave front hitting the elements at
unequal times thus being slightly out of phase. This situation really
becomes a problem at UHF because as the frequency gets higher the phase
shift is more due to shorter wavelengths. This problem does not exist in a
"stationmaster" style antenna due to the stacking of all elements on top of
one another.
I have had much better luck when using a 4 element dipole array on 2 meters
(in omni) using the DB products theory of positioning the elements in the
north south east west approach than the north east south west approach, for
what ever reason.
[Image]
A compromise to the compromise.
If a "double dipole" style exposed array is utilized, the effect of
staggering the elements is less. Why? As I have read in my "Antenna
Engineering Handbook" (by Johnson and Jasic) the concentric center of a
double dipole array is more so the -mounting pole-. The DB 304 for 2 meters
and the DB 420 style antenna for uhf utilizes this approach and are proven
performers.
You stated that downtilt may be a consideration. If the antenna gain is 5 or
6 dBd there may be no advantage in utilizing downtilt since the -3 dB point
would be fairly close to the repeater site. As antenna gains increase, there
are advantages in using downtilt. Downtilt can be achieved in two ways.
Mechanical and electrical.
Mechanical downtilt works well when a repeater only needs to work in one
direction. (multipanel arrays not withstanding)
Electrical downtilt works well in omni patterns but there is a loss in
overall gain. A typical 10 dBd stationmaster on uhf will only have 9 dBd
when built for downtilt. 10 db commercial stationmasters in the 450-460
range work great on 440 repeaters especially when vertical beam downtilt is
required. For more info on understanding vertical beam downtilt.. Go to
Top mounted fiberglass antennas suffer from precipitation static more so
than exposed dipoles do. Go to for
more info on precipitation static.
I use a DB Products 4 bay double dipole array on a 2 meter machine. I feel
there is no advantage to this antenna over a super stationmaster except for
the lack of precip. static. [Image]
My suggestion:
Since you seem to be leaning toward the dipole array, and not knowing if you
have the availability to the top or side of the tower for mounting please go
to
The DB 228 is an antenna with many advantages you are looking for. (no image
available)
There is a load of information on mounting and expected patterns on this
site. There is also a calculator and software section at
Very useful.
The DB 228 can be set up for omni directional coverage. Because this antenna
utilizes all elements in the same direction, a whole bunch of gain is
obtained. This style antenna is used on a repeater I do some work on that
the other moderator to this list operates (Dan Ruhe KE3UC) If specific
questions need answered they can be directed to either him or I through the
list.
Hope this helps,
Kevin
