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1994-05-30
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4KB
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89 lines
SUBJECT: VHF & UHF Hidden antennas for PUDs/CONDOs/APARTMENTs
Part 2
INSTALLATION OUTSIDE continued
Next, I'll discuss the 1/4, 3/4 wave ground planes. These antennas have
various gains, but all have one thing in common. Each antenna requires no
tuning or matching elements. Like the 1/4 wave, 3/4 wave antennas
are built in the same manor except the vertical element is 3/4 wave.
The Quarter wave 2m antenna can be cut so that it is a 3/4 antenna
on the 440-450 MHz band. Thus, a dual band antenna. A 1/4 wave on 222-225 MHz
can be cut for a half wave on 440-450 MHz, another dual bander. These dual
band antennas will not have the complete range over the secondary band, but
will give a piece of it at low SWR.
Each antenna will cover the entire primary band it was cut for. I use brass
rod for my ground planes. They do not rust and solder easy with a good
soldering iron.
Hiding these antenna takes a little more foresight. The ground plane shape
tends to make the posibilities of a good hiding spot a bit less. But the
thin material tends good for trees. They can be hidden as part of the tree
with plastic leaves and a blending color paint job. Their size is a good
contributing factor. UHF vertical ground planes (VGP) tend to hide themselves.
A UHF 1/4 wave VGP is a mere 9 inches from the base of the ground plane to
the tip of the vertical element. I wouldn't leave them out in the open, but
little will have to be done to hide one of these UHF VGPs.
----------------------------------------------------------------------
Discones have been used outside in restricted areas. Hiding one can present
a problem because of the dimentions. I would suggest using these antennas
indoors and will discuss them later.
----------------------------------------------------------------------
The Vertical Dipole is an easy antenna to hide. The shape is easy to bend
in with surrounding objects and matching can be accomplished by many means
as listed in the ARRL antenna handbook. My favorite is the gamma match.
I would recommend this for the 2m and 1.25m antennas. At this point, I must
say the gamma match works fine, but at UHF frequencies, a Tee match is
better. The gamma tend to loose the bandwidth required up on the 440-450 MHz
band.
It can be hidden in the same manner as the J-Pole.
----------------------------------------------------------------------
The Full Wave Loop is another favorite. Although this antenna has the best
gain of the previously mentioned antennas, it can present a problem in band
width and placement. When placed against a building, the SWR goes up.
Cutting and tuning a loop to a band may also present some problems. The
bandwidth of a loop may not cover all the frequencies desired if operating
the repeaters.
The circular loop is the best, followed by the square loop. Triangular loops
work well also. The area enclosed by the loop is the main facter in
approaching the 50 ohm impedance required by amateur radio gear today.
Please keep that in mind. Geometric shapes in a natural setting tend to
'stick out' to the human eye. Hiding the antenna will require a crafty
plan on the part of the amateur. The bidirectional pattern from the loop
my hinder the placement if a specific direction is required. Remember
the chimney? Good place to start.
----------------------------------------------------------------------
Enough for now. Next, I will discuss the VHF/UHF hidden beam antenna.
I have not discussed 6m, 33cm, or 23cm antennas, but the same application
holds.
Here's some info on 1/4, 3/4 wave lengths for the bands 6m, 2m,
1.25m, 70cm, 33cm, & 23cm to give you an idea on the size of the antennas
you will be dealing with. The formula use does not always relfect the exact
length of the antennas involed, but is to be used for sizing only.
1/4 3/4 f
------------------------------------------
6m | 4.73' 14.19' @ 52.0 MHz
2m | 1.68' 5.05' @ 146.0 MHz
1.25m| 1.1' 3.3' @ 223.5 MHz
70cm | 0.55' 1.65' @ 445.0 MHz
33cm | 0.27' 0.806' @ 915.0 MHz
23cm | 0.194' 0.581' @ 1270.0 MHz
73, Wayne KB4YLY, EMLab, KSC, FL