OK
I just read the Jim Brown power point presentation for the 3rd time now ...
very well done.
If I have it right... inductive baluns are a no no... since if the coax cable
is less than a quarter wave... the cable is capacitive,
and a cable longer than a quarter wave....but shorter than 3 quarter wave....is
inductive.
In both those cases the common mode current will increase.
OK... now what happens if the cable is EXACTLY an electrical 1 qtr wave
long..... or exactly
3 x qtr wave long.... or 5 x qtr wave long. Per Rauch... the Z at the
feedpoint should be sky-high,
and there is no need for a balun. In fact.. a typ balun will make it worse.
Rauch advocates grnding the braid of the coax at the top of the tower and or
mast asap.
Why not just make the coax an electrical odd multiple of a quarter wave..and
then bond the
braid to the tower. I`m tallking abt the case of monoband yagi`s on a mast,
above tower.
In my case... I have a 1 in..6 out remote switch box on the mast.... so only
one rotor loop.
That being the case.. ALL the braids of the coax`s for each yagi are grnded
just above the top of the
tower.. on the mast. An electrical qtr wave may well be too short... and a 3
x qtr may end up with loads
of excess coax. The concept may only work for certain yagis... depending on
band.. and how far out the DE
is on the boom.. and height above top of tower,etc.
EG... 40 m yagi is at botton of mast... so I don`t want a qtr wave of coax..
when the DE is 2 ft from the mast...
and remote switch box is only 2ft above the 40m boom.
A local here in town said he read an article a while back... where they did the
above... for each band on a
multi band quad. Each band had it`s own coax... and each coax was an
electrical qtr wave .. or odd multiple,
and no baluns were used at all. The idea there was to eliminate the weight +
bulk of 3-5 x baluns.
Buried some where.... the caveat was.... the odd qtr wave coax trick
depended on the height of the tower.
Rauch never mentioned the effect of tower height on the odd qtr wave coax
lines.
In the old days.. we used either gamma or omega matchs, NO baluns.. and no
pattern distortion.
Front to back ratio was never de-graded... nor was Front to side ratio either.
Apparently... you can
eliminate the gamma cap altogether... by shortening the DE some more.. and esp
more on the DE side that has the gamma rod.
That was in a mid 80s qst article. With no gamma cap at all... the center
conductor of the coax is also now at dc grnd.... which
is another bonus.
I was toying with the idea.. of installing a choke balun on the INPUT to the
remote coax switch box. Then, each yagi
would have it`s own, individual balun, at the DE. The braid of each yagi`s
coax would also be bonded to the mast... via the remote switch
box. Then the balun on the input side of the remote switch box should
kill any residual common mode current. Braid of coax could
also be bonded to earth grnd... at base of the tower , via cadwelded grnd rods.
Another choke balun could be installed in the basement..
where the single coax 1st enters. After that.. I give up.
Any thoughts
later... Jim VE7RF
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