Hello Contesters:
thanks to the following stations who replied to my questions
concerning matching transformers of cubical quad antennas.
DK4RM, Xaver
K5FUV, Bill
WA7FOE, Ron
K4JRB, Dave
N0AX, Ward
WA2SRQ, Ed
K5ESW, Paul
KI7M
VR2BG, Brett
KN6IS, Mike
and many other stations
The subject of my survey was: "Is a quad working without compromise
with a matching transformer?"
===> In a word: No
KI7M reported me that one thing !SURPRIZED! him about the LBA quad,
when he put it up all the bands had near perfect SWR! He has never
seen this with any other quad. It makes him want to remove the
transformer and feed the elements direct with the coax to see what
difference that might make. Just too good to be true.
K5ESW reported:
There is probably interaction. When you consider that a 20M element
will be two wavelengths on 10M, you can see why. The 20M element will
have an impedance on 10 that will be a close match, and therefore take
current.
Since all driven elements are connected together, current will flow
from the source into each element depending on the impedance at the
particular frequence of the source.
The question he has not yet answered, is what is the interraction, of
just having the elements there (not physically connected).
He also found some interraction at 12M, but not as bad as on 10M.
WA7FOE told me:
He has the 2 element 5 band Lightning Bolt Quad. He had it up for
about 1 year now. Spent the extra $50 or so and ordered the HEAVY
DUTY 5 kw Balun, and the HEAVY DUTY boom, which means I can extend to
4 elements at any time.
He uses the antenna as primary for WARC bands, and secondary contest
antenna for obscure directions the Yagis aren't pointing.
Antenna has been very good. He find I end up usinging it a lot for
general purpose DXING. As a plus, it's not too expensive. We have the
antenna mounted at 108 feet, and it has been through 67 MPH winds so
far with no problem.
N0AX says:
The way he feed his quad is an RCS-8V 5-position remote switch and
quarter-wave transformers of RG-62/U to each driven element. This has
been absolutely trouble-free (once I determined that I needed to
provide some strain relief to the solid center conductor of the
RG-62/U) and eliminates the interaction.
He highly recommends the remote switch. If you go to all the trouble
and expense of putting up a quad, don't waste it by tying the driven
elements together. A gamma match on a quad is a mechanical nightmare
as the elements are wire, not tubing. Don't be fooled by the "match
on all bands" story...it's probably a lossy arrangment...if you don't
see a normal U-shaped SWR curve on each band, then you are probably
heating up some resistance somewhere, or the antenna is 'way out of
resonance.
Regardless of the concentric or cubical arrangement, the quad is a
hard antenna to beat, especially at 60' or less from ground.
Ed WA2SRQ did some modelling, his results:
Hi Charlie,
I have recently been modelling quad antennas with EZNEC, and these are
a few things I have discovered:
- A single band 2 element quad has about 8dBi forward gain, about the
same as a 3 element yagi. This comparison done on 20m, using a 10'
boom length on the quad and my 203BAS HyGain 3-element yagi for
comparison which has a 16' boom. The feedpoint impedance is about 100
ohms.
- If you add 15m elements to the above mentioned 20m quad, you get
almost exactly the same results with very little interaction between
the bands. 8dBi on both bands, 100 ohms feedpoint impedance, even
with both driven elements fed with 1 feedline. The quad loops appear
to be high impedance on the frequencies other than where they
resonate.
- If you add 10m elements to make it a 3-band quad, the 20m and 15m
performance is practically unchanged, and the 10m performance is down
about 1dB in forward gain. On 10m there is some current in the other
elements which affects gain and front to back, but the performance
still looks acceptable, especially for a tri-band antenna with 1
feedline. SWR still looks better than 1.6 (I think, from memory)
using a single feedline and a 2:1 matching balun.
K5FUV Bills comments:
that he used a 4 element Cubex on a 30 ft boom at 70 feet for a few
years, but ALWAYs with separate feedlines. If you have any second
harmonic on 20 meters, it will be radiated quite effectively with a
common feed point. He used a switch at the top of the tower that
grounded unused feedlines. Also, with the spacing used, a 50 line
provides a fair match to the 20 and 15 meter loops (DON'T prune for
1:1 SWR), and a 1/4 wave 75 ohm transformer was used on 10 meters.
This was the most dominant antenna he ever used on 10 and 15, and
competitive on 20m. This includes all of the stacked Yagis he had. I'd
do it again, except for the combination we have up here of high winds
and ice at the same time.
VS6BG, Brett's report:
I use the Lightning Bolt 5-band quad here. I highly recommend it.
Be sure
to order it with the 3-inch OD boom - it is much more robust & easier
to work with in my opinion, especially here in typhoon country where
one must take it down before the storm hits. The design, with the
removable spreader assembly, is very good & allows me to take it down
without any help, even with heavy winds.
The reason why it must be taken down is not because it won't survive
the winds. This is one robust quad. Our problem in VS6 is the stuff
that gets blown by the wind hitting the antenna, plus the fact that
our cheap JA aluminum towers cannot support much more than themselves
when the winds go over 70 km/hr or so.
VS6WV also a 2-el Lightning Bolt. His has a single feed for all five
bands. I split the feeds for mine, with separate feedlines for 20,
17/15/12 & 10m. I did it that way so that I could work two bands
simultaneously - important for serious contesting. If you do this, be
sure to have LBA send you the matching transformer that they normally
supply with the 4-element version. This has two taps for different
impedances - handy for matching & also useful if you want to add two
more elements in the future (remember, you can never have too many
elements or too tall of a tower!).
My quad is on a 6m tower which sits on the roof of a concrete house
about 9m tall. SWR is FB on all bands but 20 - it is resonant a
little low (about 13.8 MHz) & I've been too lazy to trim it. The
other bands are like what KI7M told you. The 20m resonant point has
changed due to proximity to the roof. However, compared to a yagi
(KT-34) at the same height, I do not see so much change in the SWR as
the quad turns. This is one advantage of a quad - lower Q.
K5ESW's comment has some validity. I've always felt that multiple
antennas on a single feeder is not a good idea. That, plus the desire
to run two radios at the same time, led me to decide for split feeds.
Just tell Mike (of Mike's Auto Electric, Mr. LBA) that you want the
VS6BG version. Run three coaxes & consider a two rig setup for
contesting as the sunspots return!
Mike Cash reported:
Karl, there will always be interaction when you have conductive
material near the RF field. Most of the time that will degrade your
performance. I use a seperate 10M driven element from the 20M driven
element to help avoid the problem. If your 10M quad is not
performaning better than a 10M vertical then something is wrong with
the quad. I do not trust computer models for quad antennas. There is
not enough validation or verification of the model to real world
performance. In the yagi world it has been proven out, but not in the
quad world.
We have 100+ mph winds here in the desert. I have been where you live
and I know that your winds are also very strong. The cubex quad is
the strongest that I know of. They use a 3" boom with a very heavy
spider.
Cubex is now on the internet and also has e-mail. You can talk to Ed
Buchanan who owns Cubex direct. His e-mail address is
EBuchanan@gnn.com or his internet is http://www.cubex.com/cubex.htm
Let me know if you have any trouble with the connection.
To make a long storry short: The cubical quad antenna is a very good
antenna for Dxing and contesting. The use of matching transformers
which allows to feed the antenna with one coax cable is only a
compromise. Best performance is only given by feeding the bands with
separate feeding lines.
I hope to put up my new cubical quad soon.
vy 73 de DL6RDE/AA1KD, Charlie
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