[TowerTalk] CMC-230-5K

[Jim Brown]

On Thu,4/21/2016 8:01 AM, Jim Thomson wrote:
> Date: Thu, 21 Apr 2016 00:50:05 -0700
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] CMC-230-5K
>
> K9YC said:
> You would save a lot if you did what I recommended about 8 years ago --
> get together a group purchase and buy in quantity. I've been part of at
> least half a dozen, two of which I've organized. Appendix One of my
> tutorial lists Fair-Rite part numbers of some useful parts and lists
> several good industrial vendors who sell in quantity at great discounts.
> Remember - we need these parts not only for feedline chokes, but also to
> kill the many RFI sources that most of us have in our homes, and that
> are in the homes of our neighbors.
>
> &&&&  Points well taken.  Trying to initiate a group buy in this town of  400 hams is
> like pulling teeth,  whether its  type 31 cores or entire spools of heliax, or anything else.
> Too bad.  I gave up trying a while back.

Are you a member of any ham clubs?  A contest or DX club?  I'm sure 
there are some around you, because I know lots of VE7 contesters, mostly 
around SW BC. The group purchases I've been part of have been with big 
clubs. The first, when I was still in Chicago, included SMC members 
around Milwaukee, Chicago, and Indianapolis, and a big club in the LAX 
area. Subsequent buys have included the NorCal DX Club, NCCC 
(NorCalContestClub), and two local clubs around my QTH in Santa Cruz.

> I've also organized several group purchases of Amphenol 83-1SPs, and
> assorted audio connectors of the type we use around the shack. I'd be
> very surprised if there are no distributors of Fair-Rite and Amphenol in
> Canada.
>
> &&&  There are distributors in canada, with the  usual...price is double or triple the US price,
> plus you must buy qty, pay shipping from VE3 land to the west coast, and wait 4-8 months,
> then cross ur fingers.   I rarely buy electronics up here, except  for  audio connectors, like
> Neutrik XLRs, of which I use quite a few, and local store has exactly what I want.
> I don’t buy car parts or gun stuff here in canada either, nor specialized tools.

That's surprising.  I have heard about US/VE shipping costs. Shipping 
costs from ANY vendor can often be equal to or greater than the cost of 
the part for onesy-twosey orders. The great thing about group purchases 
is that those costs are divided between hundreds or thousands of pieces.

>   
>
> Don't you recognize an advertising claim when you see one?  I'm not
> trying to sell you anything, I publish measured data on the chokes, AND
> tell you exactly what's in them and how I measured them. Do you see ANY
> of that in the magic boxes you're buying?
>
> &&&  No offence, but hes using the same HP gear + more, as you are using to measure
> them.  If you doubt his Z / RS claims, then measure one for yourself.  Im sure that can easily
> be arranged.   Then you can verify his claims one way or another.

I have better things to do than be a free test lab for every vendor that 
comes along. It takes a LOT of time to do accurate measurements of these 
chokes, for reasons noted in my tutorials.
>
>>   
>> It's a brand new product, Jim!  And you really believe a mfr is going to
>> tell you about his failures?
>>
>> &&&&  Its  sitting in several contest stations already.  Nobody has blown one up...yet.

That's hardly a meaningful claim for a product that you're selling. And 
"sitting in several contest stations" means very little unless the choke 
is installed in a way that puts high common mode voltage across it. That 
means AT THE FEEDPOINT of an antenna with significant imbalance and 
running high power with high duty cycle.

Note that I determined power handling by putting chokes at the END of a 
half-wave dipole, where the outside of the coax is the bottom half of 
the dipole. That's a VERY high voltage point for the choke, and I ran 
high power into it.
>
>> ##  To increase power handling capability of a CMC, there is a few ways to do it, and that depends on
>> how much spectrum it has to cover. More cores + less turns is one technique.
> WRONG!
>
> &&&  Did u try it ?    IF  you can achieve similar RS, (on one band), the choke with more cores
> has more thermal mass.   But that’s  difficult to achieve similar RS on a given single band.   There are
> several different combos  not listed on your Fig-41 graph.  Like   6 turns on 7 cores.  Or all the various
> combos of 8-9-10 core stacks.    If you use a binoc config, you can increase in half turn increments,
> which allows for finer tweaking.   With a single stack, you have to use full turn increments.

The measured data clearly shows the measured Z vs frequency, which can 
be used to find R, L, and C for the parallel equivalent circuit, which 
is what a ferrite choke is. Once you've done that, you can easily 
transform to a graph of Rs vs frequency. All the data is there for you 
or anyone else to do the work, and I described how I did it. Even 
better, you can plug the parallel resonant circuit into an NEC model of 
the antenna and let NEC compute the dissipation in the choke.
>
> &&& Those 2.4 inch torroids are only 118 grams each. The big beads that slide over 213-U  are 55 grams
> each.  I suspect that the reason that the string of beads never heats up, is because of the total greater mass
> of the beads.   5 x torroids =  10.72 beads.   Typ, if beads are used,   15-40 are used.   The most I have seen used is 7’.

Again, you clearly do not understand how ferrite chokes work. Please 
study my tutorials. Those beads don't heat up because they have very 
little Rs with only a single turn. And, because they have inductance 
with very little Rs, they can resonate with the feedline, which 
increases common mode current rather than choke it. The ONLY string of 
beads choke that is effective at HF is W2DU's original design, which 
uses #73 beads, which ARE self-resonant at HF, and ONLY when built with 
200 beads to get the choking Z high enough. Unfortunately, the 
commercial product used only 50 beads, the smallest number of beads he 
tested in his published work, which results in a choke that is less 
effective and can't handle high power.

Later versions of a string of beads used #31, simply because I had noted 
that #31 was a better HF material, but failing also see that I had said 
that #31 was only useful at HF if a sufficient number of turns was wound 
to bring the resonance down to the desired part of the HF range. Those 
who didn't know any better (including popular QST rip-off resellers) 
were among those who made this mistake.

>
> &&&  each application is different.  We can only estimate CM voltage, then go from there.  A lot of variables
> in play.  Like is the ant really balanced, or slightly off balance.   Is the shield of the coax bonded to the top of the tower?
> What is the length of the coax from yagi feedpoint.....to  top of tower?   If its  .25  or  .75 wavelength, things typ get better,
> at least on one band, if a multiband yagi used, but then u have to factor in C from braid to boom.    Then factor in height
> of tower, and how well tower is grounded at its base, etc.

Yes. That's part of why my recommendations for choking Z is a "brute 
force" approach -- both for choking effectiveness and power handling.

> &&& measuring core temps, when they are way out on a boom is no easy feat.  Not an issue on a 3 el 40m yagi, with its DE next to the  tower.

That's true. But it's not necessary to make that measurement. A failed 
choke will show high SWR.  And, as I've noted, there are other ways to 
test chokes for power handling.

73, Jim K9YC