Date: Thu, 4 Dec 2014 09:39:20 -0000
From: "Ian White" <gm3sek@ifwtech.co.uk>
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Choking on chokes
Truly balanced antennas are beyond "rare". In the real world, they don't
exist at all.
There is a strong ham myth that center-fed dipole antennas in some way
"want" to be balanced; but that is almost completely untrue. Very much
like a pencil balanced on its point, a balanced antenna is a highly
UNnatural state - deceptively easy to imagine, but never seen in
reality.
So please let's stop imagining. All real-world antennas will have some
degree of unbalance caused by asymmetry in either the antenna or its
environment; or both. It follows that there will also be *some* level of
common-mode current on the feedline; it won't ever be zero. The
challenge for hams is to stop imagining an idealized state of balance
that cannot exist, and to engage with the real-world engineering
questions:
A Yagi with a center-fed driven element and installed on a tower is
probably as close as any antenna can ever come to being symmetrical and
balanced. Therefore common-mode current on the feedline is likely to be
small, and also quite easily suppressed. Even quite a poor common-mode
choke will often seem to work for situations like this - not because the
choke was much good, but because this was a "soft" problem, easy to
solve.
73 from Ian GM3SEK
## feeding a yagi with no balun at all..and just coax taped to the boom, imo,
would
result in something a lot worse than a ...”soft problem”. Balanced DE, and
unbalanced
coax = a real mess. Then you have the high C between the braid and the boom,
which will
virtually guarantee it all ends up unbalanced. . Then it gets further
compounded depending
IF the braid is bonded to the boom, top of tower, or close to the top of the
tower. Toss in a
hairpin or beta match, vs OWA, and now we have tied both sides of the DE
together.
## I cant see a yagi working with out a balun. I cant envision the system
being a soft problem either.
My best guess is... you probably require at least 1 k choke Z to not screw up
the ant pattern. And
a lot more Z, if ferrite is used, so the ferrite does not over heat, esp if
type 31 is used.
## aside from monoband yagis, most multiband yagis will consist of the
20-10m variety, or 20-17-15,
40-20-15-10m, or 17-12, 40-30 types. IE: we don’t require a balun that has
to cover 160-10m.
That being the case most of the time, it would be a relatively easy matter to
optimize the balun for the freqs
in question. IE: if you have a 40m yagi, optimize the balun for that one band.
Ditto with a 40-10m yagi,
or a 20-10m yagi. Same deal if monobanders are used, optimize for one band.
## two identical, optimized baluns could be wired nose to tail, in
series..with a bit of space between them,
and send the Z through the roof..or they could be stagger tuned if trying to
cover say 40-10m. I’m talking
about the coax through the typ 2.4 inch OD stacked cores in the above 2
paragraphs .
## Laff all you want, but 4 feet of the large type 43 beads, slid over 213-U
will provide >3 k ohms Z on 40m alone, and
a lot more Z as the freq increases. Like 4.9 k ohm on 20m..and even more on
17-10m...like 6.6k on 10m.
Dead simple, and they can be taped right to the boom. No WX enclosure
required, and they flat out wont heat up.
I usually put heat shrink over the entire mess, but even that is not required.
The beads can touch the boom, and also
sit in the rain. The boom will just heatsink them. They have too much combined
thermal mass to heat up.
## IF the DE on the yagi is perfectly balanced, or close to it, we can take
the 274 v ..... which is 1500w into 50 ohms...
and divide it in half. So only 137 V appears across the balun. 137 x 137 =
18769. 18769/3000 ohms = 6.25 watts.
With 1000 ohms, its just 18.77 watts. That’s key down RTTY. 6.25 watts is not
going to heat up 4 ft of large beads,
taped to the boom, any time soon, nor will 18.77 watts. You can do a similar
calc for higher or lower Z.....and also higher
or lower power levels. Since the resistive portion is less than the Z, the
actual heat generated is even less than calculated above.
5 kw RTTY is 20.83 watts with 3 k ohms. Each bead is 1.020 OD x .505 inch ID
x 1.125 long. The wall thickness is .2575
Jim VE7RF
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