ssd's Quad Antenna pictures
Pictured here are (so far):
All of these (so far) are tuned for 442 mhz, use a BNC feed connector,
and are designed for a high degree of portability.
- Attempted dual wire quad. This is my third or fourth attempt at a quad.
Thicker wires failed due to difficulty in shaping them accurately and several
mechanical failures, so I
thought I'd try thin ones simulating thick ones. First attempt
at this was wired a bit loose. I abandoned this design when I came up
with a better idea... I may come back to it if I see an advantage.
Second picture shows a spacer I tried soldering in. If I had finished
this, all four sides would have one. All four corners already have one,
but I forgot to take a close up of that.
- Successful three element (high gain) quad.
This is my third attempt using this construction method. Previous attempt is
the two element quad shown next.
I meant to take a close
up of my fine solder work on the corners, but again, forgot. :)
I've gotten several comments from people impressed. Personally, I think
the loops are pretty. The supports less so, but they're functional.
The black ball hanging from the bottom is a compass -- useful for untwisting
my mind around this circular campus where none of the buildings line up.
- Successful two element quad.
This is my second attempt using this construction method.
First one had some dimensional accuracy problems.
(I figured out how to line up the micrometer with the axe a little better.)
This works well, but has a high SWR due to impedance mismatch.
(Hope to fix that soon.)
Note the holes in the first support shown -- from my first quad attempt,
which used the square plastic as the beam holding
fiberglass spreaders using coax shield for the elements. (Not pictured
here yet.)
Notable differences between two and three element quads:
- In the 3 element quad, most of the support slots for the loop are
edge cut rather end cut, as I found this has greater stability.
Note dowels and skewers stuck down the shaft and through the loops.
This keeps the elements in snug, and also stabilizes them greatly.
- On the two element quad, I moved the feed point
to the center rather than the corner.
This simplifies mounting the antenna -- I can leave it on a table.
Also, this allows a fourth loop support, which is badly needed due to the
end cut slots. (This was the first iteration, remember.)
The three element antenna was designed to be hung rather than placed
on a table, partly because of its intended final destination, and partly
to make it easier to aim (er, wildly swing about).
Details:
| antenna | predicted
impedance | predicted SWR
at 4.6 ft RG58 | measured
SWR | predicted
loss | predicted
antenna gain | total gain | measured gain
|
|---|
| 2 element | 144 | 2.34 | 2-4 | 0.85 | 7.1 | 6.3 | 5-10 dBd
|
| 3 element | 49.8 | 1.06 | 1.7 | 0.59 | 9.6 | 9.0 | 10-15 dBd
|
Receive gain was measured vs. a 1/4 wave dipole.
Transmit gain is undoubtably worse, due to impedance mismatch.
I've yet to even try to match impedance on either antenna, as they seem to
work well enough without it, and my preliminary attempts to match it
made things worse rather than helping.
More recently, I've made another 2 element version of this using the
wire left over from the 2m loop: (0.128" dia).
Of course, the big difference is the bandwidth and structural stability;
the impedance and losses are about the same.
| diameter | predicted 2:1 SWR | predicted >20db front:back
|
|---|
| 0.08" | 73mhz | 15.7mhz
|
| 0.128" | 85mhz | 18.7mhz
|
I've used all three of these antennas across the 70cm band, and they
all work well. The biggest difference with the new antenna is that
it is much less fragile with the thicker copper, and I've tuned it
to 440mhz (the lowest I intend to use it) rather than 442.
Even with the mismatch, I get full quieting and <3:1 SWR.
As I use these antennas on a regular basis, I will probably experiment
with methods to get a good impedance match. If nothing else, this
might keep my HT from overwarming my hand.
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