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Sent: Tuesday, December 09, 2014 10:00 AM
Subject: TowerTalk Digest, Vol 144, Issue 14
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Today's Topics:
1. Choking on chokes (Jim Thomson)
2. Re: Choking on chokes (Ian White)
3. Re: Conduit installed in foundation slab (Blair)
4. Re: Conduit installed in foundation slab (Marsh Stewart)
5. Type 73 beads, W2DU balun. (Jim Thomson)
6. feeding a Moseley S-403 40 meter three el. beam
(John King via TowerTalk)
----------------------------------------------------------------------
Message: 1
Date: Mon, 8 Dec 2014 22:59:45 -0800
From: "Jim Thomson" <jim.thom@telus.net>
To: <towertalk@contesting.com>
Subject: [TowerTalk] Choking on chokes
Message-ID: <09E950B04434405994D36A339E5CE921@JimPC>
Content-Type: text/plain; charset="utf-8"
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
------------------------------
Message: 2
Date: Tue, 9 Dec 2014 07:52:27 -0000
From: "Ian White" <gm3sek@ifwtech.co.uk>
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Choking on chokes
Message-ID: <001301d01385$18e7ffe0$4ab7ffa0$@co.uk>
Content-Type: text/plain; charset="utf-8"
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?.
I never said that a Yagi doesn't *need* a balun!
Using no balun at all would just be a fool's way of turning a "soft"
common-mode problem into a tough one.
73 from Ian GM3SEK
------------------------------
Message: 3
Date: Tue, 9 Dec 2014 10:25:38 +0000
From: Blair <ve3zbm@rogers.com>
To: " towertalk@contesting.com " <towertalk@contesting.com>
Subject: Re: [TowerTalk] Conduit installed in foundation slab
Message-ID: <186875.92783.bm@smtp231.mail.bf1.yahoo.com>
Content-Type: text/plain; charset="utf-8"
Thank to everyone who replied to my query. You have asuaged my fears and
will proceed with embedding my feedline conduit in the tower foundation. I
will likely cap the schedule 80 conduit with a termination similar to that
which is used for overhead electrical utility lines feeding a meter base.
I can?t recall the actual name for this cap at the moment, but you get the
idea.
Regards, Blair.
VE3ZBM
On 12/08/14, Blair (VE3ZBM) wrote:
Good afternoon, All.
I have not found a satisfactory answer to whether or not a 2 or 3"
schedule 40 PVC conduit, installed ~3' below grade and into a 5' x 5' x 4'
deep steel-reinforced free standing tower foundation will derate or harm
the slab's overall integrity. I am preparing for my new tower (to be
installed next spring) and would like to bring the cable feeds up inside
the tower base thereby eliminating tripping hazards and other aesthetic
concerns.
Thanks in advance,
Blair VE3ZBM
Sent from my iPhone
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Message: 4
Date: Tue, 9 Dec 2014 06:49:42 -0600
From: "Marsh Stewart" <marsh@ka5m.net>
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Conduit installed in foundation slab
Message-ID: <A5E7B55FE4D54D64ADBAE8175BC6B49F@i7MarshSept13>
Content-Type: text/plain; charset="us-ascii"
The electrical device you are referring to is called a "weatherhead".
Marsh, KA5M
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Blair
Sent: Tuesday, December 09, 2014 4:26 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Conduit installed in foundation slab
Thank to everyone who replied to my query. You have asuaged my fears and
will proceed with embedding my feedline conduit in the tower foundation. I
will likely cap the schedule 80 conduit with a termination similar to that
which is used for overhead electrical utility lines feeding a meter base.
I
can't recall the actual name for this cap at the moment, but you get the
idea.
Regards, Blair.
VE3ZBM
On 12/08/14, Blair (VE3ZBM) wrote:
Good afternoon, All.
I have not found a satisfactory answer to whether or not a 2 or 3"
schedule
40 PVC conduit, installed ~3' below grade and into a 5' x 5' x 4' deep
steel-reinforced free standing tower foundation will derate or harm the
slab's overall integrity. I am preparing for my new tower (to be installed
next spring) and would like to bring the cable feeds up inside the tower
base thereby eliminating tripping hazards and other aesthetic concerns.
Thanks in advance,
Blair VE3ZBM
Sent from my iPhone
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------------------------------
Message: 5
Date: Tue, 9 Dec 2014 08:02:15 -0800
From: "Jim Thomson" <jim.thom@telus.net>
To: <towertalk@contesting.com>
Subject: [TowerTalk] Type 73 beads, W2DU balun.
Message-ID: <BBDFF71C004646C385118B03F11C834F@JimPC>
Content-Type: text/plain; charset="iso-8859-1"
Here is something interesting. The type 73 beads used in the W2DU balun
are pn 2673002402 per page 26 of the K9YC tutorial.
Each bead is .380 OD x .197 ID x .190 long. Each bead weighs a mere
1.2 grams. 50 beads are used...for a total length of 9.5 inchs.
Total weight is 60 grams... or .132 lb. His experimental design used
300 of em.... or 360 grams = .793 lb, and 57 inchs long. .
The type 31/43 beads that slide over 213-U are 1.020 OD x.505 ID x
1.125 long...and weigh in at a hefty 55 grams each. The same 9.5
inchs long of the bigger beads would weigh 1.02 lb.
Big difference between .132 lb and 1.02 lb ...as in 7.7 times
heavier..for the same length. More thermal mass too.
Just an observation as I have not noticed the weights of the various beads
before.... nor the 2.4 inch od toroids, which
weigh in at 118 grams. One 2.4 inch toroid weighs the same as just 2
of the large beads.
Jim VE7RF
------------------------------
Message: 6
Date: Tue, 9 Dec 2014 08:36:31 -0800
From: John King via TowerTalk <towertalk@contesting.com>
To: towertalk@contesting.com
Subject: [TowerTalk] feeding a Moseley S-403 40 meter three el. beam
Message-ID:
<1418142991.33546.YahooMailBasic@web160305.mail.bf1.yahoo.com>
Content-Type: text/plain; charset=us-ascii
I am putting up an S-403 Moseley three element 40 mtr. beam. Does anyone
have hands on experience with this model beam ? It has a resonating coil
in the center of each element. The manual discusses feeding directly with
coax OR with a multi-turn RF Choke made of coax. I have used both feed
methods on antennas in the past. But with this installation, I would like
to use a commercially manufactured 1:1 torroid balun of W2FMI design
rated at 5KW . The manual does not suggest use of a balun, but does not
discourage the use of a balun. My question has to do with the feed point
which has a resonating coil at the center of the driven element .
If you are familiar with the S-403 and have experience with this design, I
would like to hear from you regarding the use of a balun between the coax
and the feed point. Your input based on your knowledge and experience will
be greatly appreciated. Thanks and 73, John, K5PGW
------------------------------
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End of TowerTalk Digest, Vol 144, Issue 14
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