<< In a message dated 8/14/01 6:04:47 PM Pacific Daylight Time,
coneal@ma.ultranet.com writes:
<<
So, the question: Would an 80M inverted V a few feet below the C3XLD have a
negative effect on its pattern/swr? The C3XLD consists of three end-to-end
"monobanders", with three separate baluns and feed lines and no traps,
sleeves, or loading gimmicks, which is why I'm hoping I will like it.
[Chuck O'Neal] Try it. You probably will find that on one band
there may be some interaction. Characterize the CX3LD by measuring
F/B with a local ham on each band before putting up the 80M
inv V. Then measure F/B with the 80M ant up to see if there is any
difference. Measuring SWR probably won't hurt either, but I have seen
significant degradation without a noticeable change in SWR. Ten
years ago I did this to answer questions about stacking monobanders
for different bands.
I put up one at a time and measured the F/B over frequency and attempted
to measure relative gain. Then I added the next antenna and did the same for
both antennas. Then the same for the third antenna. Yes, there are
significant affects on the higher frequency antennas by the lower
frequency antennas. Tks for all the thoughtful and interesting inputs!
73, Jerry
>>
Jerry: This is a "very informative post". The only thing I can add is that
a practice of adding guys to the "top of a tower" is often "totally
unnecessary." If you can't connect metal or even Phillistand guys 10-15'
below the top of a tower, it's on the weak side for the average beam. (Old
K7GCO Axiom) I've used towers and beams where the guy wires could be 20'
lower. It sure makes it easier to install a beam around them. I've often
used 2 beefed up inverted vees as the guys also. It's seems totally absurd
to run inverted vees inbetween metal guys as they affect the inverted vee and
can really excite the tower.
Typically guys often attach at the tower joints. If inbetween it may be
necessary to beef up the connection point. If it's a big beam use
Phillistrand connected to the top of the tower and for a minimum of 1/2 wave
of the beam frequency. Eznec simulations and actual tests show metal guys
below this point are virtually immune to the beam regardless what those say
who have never tried it. You can often make a 80/40M slanting vertical
starting from the guy connection at the ground. Install radials at that
point if 1/4 wave verticles are used. On high towers 1/2 wave verticals are
possible center fed and no radial system is needed or end fed with an L
network at the base and a ground rod to cool the coax shield. It can be made
into a 4 square of sorts--even 2 bands. The concept of driving 1 slanting
guy wire and using the others as parasitic reflectors works well. I will do
more on all this--when I get time. The techniques above make for more use of
the guy wires which has never been promoted for unexplained reasons?
Another possible "semi inverted vee" configuration I've described many
times is to start it at a point on the tower on the same level as the "1/2
wave Phillistrand" connection to the metal guy wire to ground and use the guy
wire to an insulator as the rest of the inverted vee.
I never have connected an inverted vee closer than 10' below a beam as I ran
the F/B and SWR checks also back in he late 40's. As Jerry points out, no
affect on the beams SWR doesn't mean that there won't be some affect on the
F/B which is easy to check. Measure the F/B in different directions. If
there is some affect on the Beams SWR, lower the Inverted Vee 5' more and
repeat the test.
<< For monoband antennas I preferred 1/4 wave coax "bazooka" baluns. They
widen
the B/W of antennas, also work as filters/stubs on single band and take
power
without frying anything. Yuri, K3BU
>>
This Bazooka shown in Collins Instruction Manuals some 50 years ago is the
best of the baluns as explained above and cheap to make. The deficiencis of
some present day baluns also tend to light up the tower with RF and they can
get warm enough to--light up also.
<<Back in my novice days (early 50's), we (hams in general) used to
use a lot of "twisted pair" for feeding antennas. Stick up a
wire cut somewhat close, hook up the feeder and go on the air and
talk to people. What was the impedance?? Who cared! What was
the SWR? Who cared! It worked and we worked guys and thats all
we needed to know. 73 Tom W7WHY
>>
This is true Tom but there weren't any SWR bridges specifically for "Twisted
Pairs" as there is for coax so what you don't know doesn't always hurt you.
Even with higher SWR values on the Twisted Pairs, transfer of power from the
final was relatively easy into a link coupled final tank of the day or the
Pi-networks later. The Twisted Pair had it's losses but everyone had the
same ones.
With open wire line there weren't any SWR bridges for it initially either
other than the "300 Ohm Twin Bulb SWR Indiator." It was made with a strip of
about 1' of 300 ohm ribbon and 2 pilot light bulbs but could be also used on
the 72 ohm Amphenol feedline of lower loss of the day which was a take off of
the Twisted Pair or even 600 ohm line. SWR was never a loss problem with
open wire line as it only changed the Z along the line. By using "certain
lengths" either a Hi/Lo-Z occured at the end of it and the proper tuner
configuration was used to match it. They knew how in those days--some still
don't today. Whatever SWR on the short feedline between the link of the
tuner and the final amp was not a problem if a series Xc was used in one or
both links. I may have to drag you kicking and screaming into the early last
Century for better and far cheaper RF transfer techniques. The "so called
modern rigs" with 50 ohm outputs unfortunately require SWR's less than 1.5:1
for effecient operation. Don't ignore SWR--know what it is and put it to
work for you when you can. k7gco
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