<< In a message dated 2/24/01 4:24:20 PM Pacific Standard Time,
w9ol@billnjudy.com writes:
< 'm seriously antenna theory challenged.
Can you stack different type of beams.
Say a log periodic on top and a Mosley ta53 below it.
Or vice versa and how much spacing would be necessary.
Not planning on anything near term, just curious.
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A clear conscience is usually the sign of a bad memory.
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Bill H. in Chicagoland
w9ol@billnjudy.com FireBrick@telocity.com w9ol@telocity.com
>>
Bill:
The rule of thumb is about 5/8 wave spacing for the gain typical of 3
element beams. The higher the gain of the beams, the narrower the vertical
patterns, the less affect each one has on each other when used individually
(at any spacing) and also the wider the spacing needed to develop maximum
gain (without ears) which also reduces the affect on each other when used
alone. This is easily seen in Eznec. When I get time I will show examples
of all this in an article. So you have to pick a compromise frequency and
spacing for max performance. Stacking 2 element beams certainly works but
due to their lower gain they need closer spacing before the ears start to
form in the vertical pattern. This simplifies tower and rotator requirements
but there is more affect on each beam on each other of the vertical pattern
when used individually.
You can stack any 2 antennas. You need equal length feedlines that
parallel in the shack for say 25 ohms and a simple L Network to raise it back
to 50 ohms. The L network has very good band width and the least loss of any
matching circuit I've found and the same configurations can be used over a
wide range of frequency if it has enough inductance and capacitance. I've
found typical rotary coils and a variable 100 uufd Xc (a parallel fixed 100
uufd Xc might be needed on 40M) are all you need to obtain 1:1 SWR on any
band like 40-2M and any where in the band. Adjust for 1:1 SWR in the middle
of the band and only on the very edges you may have to do a slight
readjustment for 1:1 SWR. It's a great concept that I and others have used
for years and haven't found tuned circuit more efficient (no heat), as
broadbanded and as inexpensive. If you visit the Flee markets you can get
the rotary coil and 100 uufd variable of about 1/16'' spacing for about $10.
I suggest having the variable inductor across the coax to the xmitter and the
variable Xc in series with the lead to the parallel connected coaxes.
Reversing them works just as well. Sometimes one orientation gives more
bandwidth. The elementary L network (2 components) is really ideal for
stacked LP's. The length of the feedline in the LP also is a phasing factor
if the other bean isn't a LP. The L network will give the widest bandwidth
of any matching circuit where ever tuned and 1:1 SWR on any frequency with
other than 1:1 SWR from the 2 coaxes. It don't care. It delivers the
desired matches even with complex loads and reactance doesn't change the
efficiency as it does with some baluns. There is a value of inductance and
capacitance that will give 1:1 SWR on any frequency. It's that simple.
Everyone that has used it loves it.
To speed up and simplify the L network tuning, connect the shaft of the
variable to the variable inductor and this will give you a "One Knob Speedo
Tuning" technique. Anyone with any mechanical ability should be able to make
this in about 15 minutes. If you can't you will be spending a lot of money
having others doing things for you. NOTE! For every turn of the inductor the
capacitor goes through its range twice and gives one an "infinite number of
impedance matching combinations" in this case below 50 ohms--very quickly.
It can tune out large amounts of either reactance along with matching any R
load below 50 ohms with the configuration suggested. With a turn counter you
can calibrate it for frequency and never have to retune on the air. If you
are on the band edges or have moved a fair distance in frequency, you can
readjust it in seconds which is ideal for contests. I'm going to prepare an
article on this circuit for one of the mags as many have asked me to. It can
be in a metal box but doesn't have to be. It's the "Ultimate in Simplicity"
and smaller components can be used than you might think.
There is another circuit for one band use that allows 1:1 SWR if both
coaxes have 1:1 SWR. Have equal length coaxes into shack preferably of
multiples of 91' 2" for .66 VF coax as this length is a 1/2 wave or multiple
on 80-10M. It repeats the Z it sees at the antenna to the feed end. If you
are using only 20-10M you can use multiples of 45' 7". Connect 1/4 wave
stubs of 70 ohm RG-59 coax for 1 KW to each feedline and combine in a Tee
connector on the other end. Connect to 50 ohm coax there to the rig. This
combo will give an even wider bandwidth than either antenna alone due to the
opposite reactance canceling affect of the 1/4 wave stubs.
There is the desirability of using either antenna alone for the different
angles of radiation. There is a "sneaky way" to connect this to a coax
switch so you can select either antenna alone or both with 1:1 SWR. This is
used by many with stacked beams but it's for one band use. Or it could be
used on stacked beams for 40&15M. The 2 RG-59 70 ohm 1/4 wave stubs are
inexpensive and Flee Market coax connectors and a Tee can get you by for
another $10 minimum with good shopping.
These are samples of simple, efficient and effective circuits that do big
jobs for a low cost. I see the need for and I'm going to start a "Retiree
Skin Flint (Tight Wad) or Retiree Metamucil Web Site" that has all these
simple, effective and inexpensive circuits. It's called "Hamming On a
Budget." It will be for the "Technically Challenged" also--good instructions
a lots of good pictures. The cost of a competitive Ham station has got so
high that the average Ham cannot afford to spend what's needed. After the
average Retiree pays his medical and pill bills he doesn't have any money
left over for Ham Radio. I use many of these techniques and have a very
competitive signal on the air without big arrays. I could do even better
with more hardware but can't install it at this QTH. There are ways to even
improve the bigger arrays commonly in use now.
I've managed to hoard a large inventory of very inexpensive Flee Market
parts of magnificent coils, capacitors, RF ammeters, insulators and aluminum
tubing and I can build anything at minimum cost (I just store the stuff to
keep those damn hoarders from getting it). I've done a lot with various
tuners I've built and have some effective variations. They handle higher
powers than the size of the component would normally dictate--no heating
and/or arcing and 1:1 SWR. You don't need large expensive components.
You can run the 2 coaxes into a balanced tuner like a MatchBox or the
"Balanced L tuner I have on one shaft. Reverse the feedpoint phasing at one
beam or have one feedline 1/2 wave longer. The Balanced L has a great
characteristic. On a 40M horizontal quad loop fed with a 1/2 wave of open
wire line the Balanced L had a reactance change opposite to the antenna that
gave a 1:1 SWR way above and below the band. That was something to see.
This is a very desirable characteristic I haven't not seen with any other
tuner.
If you are stacking different beams I'd have the higher gain beam on top
and the lower gain beam on the bottom for a variety of design reasons. More
on this later. K7GCO
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