I am planning 5 masts for an irregular pentagon to be fed
with ladder line to a Johnson Matchbox (275W). The available
perimeter is somewhere between 520 - 560++ feet (there is a
bit of adjustment available on the exact placement of one of
the masts, the other masts are fixed. I want to use this
antenna for multi-band use and am not sure yet exactly what
precise frequencies I might be on. Since I will hoist up each
point with a pulley and dacron rope the excess between the
total wire length and the actual perimeter (call it "D") will
allow a certain distance away from the metal masts which I
assume is desirable. My question is: what would be a good
precise antenna length to shoot for given the intended
multi-band use AND taking into consideration that I can
utilize "D" for spacing the antenna wire away from the metal
masts a few feet at each facet?

(1005/3.8)x2 = 529 feet, ok, but is (are) there other
length(s) that might have some obscure magical mystery
properties I may be overlooking. Obviously I am more
interested in 75-80m band than 160m per se.

I though I heard somewhere that calculating for the "lower"
part of the 80m band had some advantage, but I cannot fing
that reference. Also, once I determine what part of 75-80m to
calculate for, do I just double it to be in the ballpark of
my available perimeter?


Bill K6TAJ

My question is: what would be a good
precise antenna length to shoot for given the intended
multi-band use

Since you are using a tuner, all you need to do is insure the impedance of
the antenna system is within the matching range of the tuner at the frequencies
of interest. The simplest way is to put the antenna up and then insure that
you can tune on the bands you wish to work. If one band presents a problem,
then change the feedline length a few feet.
You could model the antenna and do an swr sweep, and that would tell you if
there were potential problems, but the tuner is a problem solver as well as
changing the line length.
The ARRL Handbook had a list of "feedline lengths to avoid" or something like
that when using a 135' dipole for all band use. As far as I know there has
never been a list for a 530' loop.
BTW, according to Reg Edwards G4FGQ above 21 MHZ your tuner will start acting
funny because of stray inductance and capacitance. I.E. the calculated values
if L and C to achieve a match will be off because of strays. I agree with Reg
on the many tuners I have built.
73 Gary N4AST

Bill,
Unfortunately there are no 'magic' lengths for your
loop. Something in the neighborhood of 520 feet should
work well on 160 meters, 250 - 260 feet for 80 meters.
The limiting factor is the 'MatchBox', it doesn't have
the impedance tuning range other tuners have because of
the small number of inductance taps it uses. (It's 'big
brother' has the same limitations, by the way.) The
'MatchBox' being a real 'balanced' tuner is nice, but just
not that big'a deal for all practical purposes.
In general (meaning that there are always exceptions) the
largest loop you can get into the air is the way to go, if
you cn 'match' it...
'Doc

I shall give it a try and see what happens. Is what you mention
the reason some homebrewers build there own matching networks?
Man, in the old days I remember there were a lot of those large
var. caps. in the ham shacks, but not anymore.

Palstar has a nice unit, but we are talking much bigger
investment. Basically, I am just putting up the antennas which
I can get away with given the boundry conditions at the qth.
Whatever I get, I am sure will be interesting. At this point, I
can, at various points here put up the loop, and up on the hill
in another "shack" I can probably fit in a double extended Zepp
(again with the ladder line). If I can get my archery chops
perfected there is still the possibility of a 160M OCF into a
tall Eucalytus tree and spanning to a mast on the side of
another chicken house. Each antenna situation, will be like an
expedition to another little shack on the property, could be
fun.


Bill K6TAJ

'Doc wrote:

Bill,
Unfortunately there are no 'magic' lengths for your
loop. Something in the neighborhood of 520 feet should
work well on 160 meters, 250 - 260 feet for 80 meters.
The limiting factor is the 'MatchBox', it doesn't have
the impedance tuning range other tuners have because of
the small number of inductance taps it uses. (It's 'big
brother' has the same limitations, by the way.) The
'MatchBox' being a real 'balanced' tuner is nice, but just
not that big'a deal for all practical purposes.
In general (meaning that there are always exceptions) the
largest loop you can get into the air is the way to go, if
you cn 'match' it...
'Doc

Bill,
I'm not familiar with the 'Palstar', so can't say how
'suitable' it would be. There are a number of the 'older'
(used/cheap) tuners that should work fine, though. I have
an old Dentron 'MT-3000' that will match almost anything
you can hook to it, for example. Something of that nature
wouldn't be quite the inve$tment of a 'Palstar' (maybe?).
You're right, there used to be many more of the 'bread-
slicer' sized variable capacitors around than now. They
were used for a lot of different applications where high
voltages were present (as in matching 'odd' impedances).
You also have other options, changing feed line lengths
to the loop for various bands. Not exactly the easiest
thing to do, but it certainly will work. Or, experimenting
to find a 'magic' feed line length that is a good compromise
for multiband use. Ask Cecil about that...
'Doc

PS - If you happen to run across a dump truck load of those
old 'bread slicers' I would gladly take them off your
hands...(he said with $$$ dancing in his eyes).

Doc,

If a rig has automatic antenna tuning, even like the older
Kenwood TS-850SAT, how does that usually work if you have a
balanced feed line? Would you still use a balanced antenna
tuner in addition, or would you override the auto tune in the
radio, or would the radio be able to handle it?

PS, concerning murphy's law or somesuch, I have been waiting to
put my coax from the 80m OCF, in some conduit through the field
to my house. I even said to myself "watch out while you mow",
but there is something hypnotic about the mower and me being
slightly a geezer (Doh! as I slap myself on the head), sure
enough, I mowed right over it, so now I have to run an
extension cord out to the field so I can do some soldering, I
assume they make a female to female bulkhead connection or
something, I will check at the local electronics store
tomorrow, of course those connectors will not fit inside the
conduit I have...etc. Sometimes I think I have the short term
memory of a gnat.

'Doc wrote:

Bill,
I'm not familiar with the 'Palstar', so can't say how
'suitable' it would be. There are a number of the 'older'
(used/cheap) tuners that should work fine, though. I have
an old Dentron 'MT-3000' that will match almost anything
you can hook to it, for example. Something of that nature
wouldn't be quite the inve$tment of a 'Palstar' (maybe?).
You're right, there used to be many more of the 'bread-
slicer' sized variable capacitors around than now. They
were used for a lot of different applications where high
voltages were present (as in matching 'odd' impedances).
You also have other options, changing feed line lengths
to the loop for various bands. Not exactly the easiest
thing to do, but it certainly will work. Or, experimenting
to find a 'magic' feed line length that is a good compromise
for multiband use. Ask Cecil about that...
'Doc

PS - If you happen to run across a dump truck load of those
old 'bread slicers' I would gladly take them off your
hands...(he said with $$$ dancing in his eyes).

Bill,
Not all 'auto-tuners' are built equally. Meaning that
the '850's tuner may or may not work with balanced feed
line. (I have two radios with auto tuners, neither will
handle the balanced feed line, even using a balun. One of
them is an '850', best radio Kenwood ever made!). It
depends on what the range of impedances the tuner is able
to handle, and what the antenna system presents to it. It's
certainly possible, but I wouldn't count on it. If money were
no object, and if I had the required building skills, I'd
have a 'balanced tuner' that would handle anything I could hook
to it. But! My 'lazy' out weighs my desire, and to be honest,
I can't tell the difference in not using a balanced tuner. (My
loop isn't anything near 'optimum' in where it is and how high
it is, by the way, so using a balanced tuner just isn't going
to make much difference. It does what I ask of it, so who
cares.)
'Doc

PS - There are only two kinds of people who use feed lines in/on
the ground. Those who have run over it with a lawn mower, and
those who will run over it with a lawn mower...

Doc,

Coax to 80m Windom is now repaired (carefully spliced together) and
now entirely in conduit and water tight. Tomorrow I will bury it in
the ground, I learned my lesson about mowers.

At this point the skywire loop is a project in process. One mast is
up, four more to go. It will probably take most of the summer to
finish this antenna. I also have another antenna in the works, and
today I got a string at the local archery shop, if you get my
drift. This one, a 160OCF will be hoisted at one end to a rather
tall Eucalyptus (100' plus) and will span across a field about 270'
to another 40-50' mast. This antenna will have to be used in a
portable shack out in the field since it is too far from the house
to run a cable. As you can see I am at that point where I am a bit
obsessed with antenna farming. If all goes well I will give the 80m
Windom (OCF) a try this weekend and will report back with the
experience.

Meanwhile I am thinking ahead about radios and the Knwd 850 did
look like a good one, as you affirmed. I see that they had a
TS-850S and a TS-850SAT, the latter being the one with the built in
tuner. I was just wondering if the SAT was the one to shoot for or
if the model without the tuner was equally worthwhile to pursue.

Bill K6TAJ

Bill,
The plain old '850 is a very nice radio. If I had
my 'druthers', any radio I bought would have an auto-tuner
built in. May not use it, but having the ability is always
nice, and you never know...
'Doc