[TowerTalk] Mosley S-402

[Jim Thomson]

Date: Tue, 28 Mar 2017 10:54:53 -0400
From: john at kk9a.com
To: Billy Cox <aa4nu at comcast.net>
Cc: TowerTalk at contesting.com
Subject: Re: [TowerTalk] Mosley S-402

I just looked at a photo and was I surprised to see that you are correct,
the elements are loaded right at the boom.  Isn't this the least
efficient spot for a loading coil? 

John KK9A 

##  If you put the loading coils half way out each ele half, their required inductance is double
what it would be if placed on either side of the boom.   For a REF, the 2 x small coils
can then be morphed into just one coil, across an insulator.  The advantage of placing the
coils out half way is the Z at the feedpoint is almost double.  F12s LL 80m rotary dipoles,
the 68 ft long ones, like the EF-180B, only had an 18 ohm Z.  F12 tried to make a 2 el
yagi, by using 2 of em, but as soon as you place a REF behind the dipole to make a 2 el
yagi with it, the Z drops way down, like down to 8-10 ohms,  so you lose it right there. 

##  If you place the coils out even further than half way on each side, their required uh values skyrocket. 
Flip side is,  to make a high Q coil, and place it half way out each ele half of say a f12 180B,  requires  aprx
a 30 uh coil, which is massive, when made from .25 inch  cu or Al  tubing.   And if you try and use smaller material
for the same 30 uh coil, say  10 ga Al wire, on a PVC form, the coil Q  goes to hell..and you still lose. 

## These days, a compromise is being implemented, with the loading coils being out only aprx  a quarter of the way
on each side.   Both optibeam, and also JK ants do this on their 40m coil loaded yagis.   F12 on their 340N  3 el, 40m yagi,
placed the LL insulator only 7ft, 2 inches  out either side if the boom. I replaced the oem LL rods with hb 2.5 uh coils, made from 
.375 inch  silver plated cu tubing, and located them at the same 7 ft 2 inch locations as the oem f12  LL insulator.  The 340N eles
range from aprx 52-59 ft long, hence the small coil values. 
JK ants  uses solid, stiff  .25 al wire for their 40m air wound coils..then he welds em where they pass through the Al standoffs..that
are in turn, bolted to the ele, and through the solid fiberglass insulator. 

##  Mosely can get away with a smaller coil, being at the center.  Then you can also make it from bigger material, and hang it from the
insulator.   Mosely link coupled the de to the 50 ohm feedline.   You still end up with a compromised, non optimized  loading scheme.
They really need to re-design it.  

## what else that works good, and is the closest thing to a free lunch is capacity hats, and they dont have to be at the extreme ends of the
ele either.   f12 called em T bars.  I replaced the oem f12 LL wires on my 180B rotary dipole with 17.5 ft long T bars, and only 15 ft out on
either side of center.   Each 17.5 ft long ele  weighs just 2.5 lbs, and are good for 124 mph winds, ice loading etc. Capacity hats are essentially
lossless.  JK ants now makes both a 2 el 40m yagi using T bars..and also a 80m rotary dipole, also using T bars. 

##  I added the seco systems tornado drive to the feedpoint of the modified 180B  dipole.  Just a pair of compressible coils, made from plastic coated
.25 inch copper tubing.  Each coil has a range of  6-12 uh.  That does the balance of the loading, and allows for 1:1  swr across the entire 80m band, like 
3200-4100  khz.   But this is a one off application, which is suitable for shortened 80m dipoles, yagis, and verticals...and also stuff like  160m verticals.  

Jim   VE7RF