[TowerTalk] TowerTalk Digest, Vol 144, Issue 14
Denis Coolican
coolican at telus.net
Tue Dec 9 21:34:16 EST 2014
----- Original Message -----
From: <towertalk-request at contesting.com>
To: <towertalk at contesting.com>
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 at telus.net>
> To: <towertalk at contesting.com>
> Subject: [TowerTalk] Choking on chokes
> Message-ID: <09E950B04434405994D36A339E5CE921 at JimPC>
> Content-Type: text/plain; charset="utf-8"
>
> Date: Thu, 4 Dec 2014 09:39:20 -0000
> From: "Ian White" <gm3sek at ifwtech.co.uk>
> To: <towertalk at 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 at ifwtech.co.uk>
> To: <towertalk at 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 at rogers.com>
> To: " towertalk at contesting.com " <towertalk at contesting.com>
> Subject: Re: [TowerTalk] Conduit installed in foundation slab
> Message-ID: <186875.92783.bm at 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
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
> ------------------------------
>
> Message: 4
> Date: Tue, 9 Dec 2014 06:49:42 -0600
> From: "Marsh Stewart" <marsh at ka5m.net>
> To: <towertalk at contesting.com>
> Subject: Re: [TowerTalk] Conduit installed in foundation slab
> Message-ID: <A5E7B55FE4D54D64ADBAE8175BC6B49F at 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 at contesting.com] On Behalf Of
> Blair
> Sent: Tuesday, December 09, 2014 4:26 AM
> To: towertalk at 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
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________
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>
>
> ------------------------------
>
> Message: 5
> Date: Tue, 9 Dec 2014 08:02:15 -0800
> From: "Jim Thomson" <jim.thom at telus.net>
> To: <towertalk at contesting.com>
> Subject: [TowerTalk] Type 73 beads, W2DU balun.
> Message-ID: <BBDFF71C004646C385118B03F11C834F at 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 at contesting.com>
> To: towertalk at contesting.com
> Subject: [TowerTalk] feeding a Moseley S-403 40 meter three el. beam
> Message-ID:
> <1418142991.33546.YahooMailBasic at 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
>
>
> ------------------------------
>
> Subject: Digest Footer
>
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> ------------------------------
>
> End of TowerTalk Digest, Vol 144, Issue 14
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