[TowerTalk] Force 12 loading coils

[Richard (Rick) Karlquist]

On 1/5/2016 3:00 AM, Jim Thomson wrote:
> From: "Peter Voelpel" <dj7ww at t-online.de>

> With wider turn spacings, capacity between turns is lower and coil Q higher.

This statement, often heard, makes intuitive sense.
Unfortunately, it is simply incorrect.  The capacity
between turns theory was debunked by Medhurst in the
1940's.  The capacitance that matters is the capacitance
from the coil as a charged body to free space or
to nearby conductors.  What turn spacings actually
have to with loss is that they determine the "proximity
effect" which increases conductor loss.  This occurs,
not due to capacitance, but due to current crowding,
where the current preferentially flows in the gap
between turns.  For lowest loss it should be uniformly
distributed.  This is also why you want tubular windings,
not flat windings.  As Jim (below) eloquently points out,
wider turns spacing is bad in every other respect than
proximity effect.  There is an overall tradeoff between
the various effects that is such that there is a wide
variation in spacings that result in a Q within a fairly
narrow range.  The optimum depends on length to
diameter ratio, but is in the neighborhood of 1 turn
spacing.  I have measured a lot of coils close-wound with
enameled magnet wire, where the turns were actually
touching.  The Q of these coils is surprisingly high,
not much worse than "optimum" spaced coils.  OTOH, the
Q rapidly drops off as you get to spacings beyond
2 turns.  If you want to read more, download a free
copy of the Radiotron Designer's Handbook and read
the chapter on inductors.  You can double check
these classic analyses with the hamwaves calculator.
I have done that and found that Medhurst, et al,
were pretty much on the mark for all but the corner cases.

>
> Centre loading is more efficient then base loading on shortened antennas.

Jim (below) is again eloquent in explaining why this
is not a free lunch.

>
>
> Silver plating of coils with cool-amp is for the birds.

Silver plating prevents the formation of copper oxide.
It is worth it for that reason alone.

>
> 73
> Peter
 >

Now for Jim's comments on Peter's posting:

>
> ##  With wider turns spacing, UH  drops like a rock.   Then you have to add a helluva lot
> more turns to get the UH  back to where it was originally.   Then you just added

Great explanation, Jim.


> ##  The problem with center loading...half way out each ele,  is the UH has to be DOUBLE vs
> loading next to the boom.   The advantage of loading half way out is the Z is higher.

Another great explanation by Jim
>
> ##  where I used 4 x SPDT vac relays  was at the feedpoint of the 40m yagi,  DE only. That provides for   4 x
> segments, which is ample..and overkill,  to ensure  flat swr across the entire  40m band.  Each coil is just 7 turns

Vacuum relays are really only needed if for T/R switching,
where you need fast switching.  Omron MJN relays are
usually adequate for this purpose.  I have used the
same multiple relay scheme to cover the whole 80/75 meter
band.  Works perfectly.  Way simpler than motor driven
caps or coils.


> ##  I silver plate all my  cu  tubing, strap, etc, simply  cuz I hate the look of bare copper...drives me nuts..esp inside a linear amp.
> I have loads of  cool-amp goop so no big deal to use it.   I have also used cool-amp  silver plating  on mech relay contacts..like the
> 30A P+B and deltrol  relays..makes a HUGE  difference.  F12  supplied deltrol 30A  DPST relays with its  40m switch box, and
> the damn contacts are high resistance, right out of the box..which makes for bad news on RX.   I have also used it on contactors,
> same deal.  We used it at work in the telco for cu buss bars..where they  bolt together..and that’s for –52 vdc  use.   Power

This is really interesting.  I have never heard of this
being done.  But I thought that silver contacts are no
good for receive applications, unless you regularly
transmit through them.  I think I'll get some of
this stuff and try it out.

>
> later... Jim    VE7RF

Keep posting, Jim.

Rick N6RK