[TowerTalk] OWAs or Fans?

[Roger (K8RI) on TT]

75/80 meter slopers are very different than those on 40 and have a much 
greater imbalance.

The reason why I'm considering so many cores is experience with a half 
wave, center fed, 80 meter fan which though it's a center fed dipole, 
it's highly unbalanced.
http://www.rogerhalstead.com/ham_files/AntennaFeed2.htm When I count 
passes through the core it's actually 7 rather than 6 turns.
With that set up, it worked well at 200 W, but as I increased the power, 
around 1 KW (maybe a little less) every LED in the system lit up. 
Instead of hanging more weight at the feed point, I added another choke 
with 5 cores and 6 turns ( wound much tighter) where the feedline 
reaches the tower. That completely cured the problem up to full power out.

Because of experience and Jim's tutorial, I'm a bit skeptical of the 
really high common mode resistance claims.  5,000-7,000, I can believe. 
The photo shows two chokes, with different number of turns to get the 
frequency range which is understandable.  I only need one octave so one 
stack, with 6 or 7 turns looks good.  I could wind a single stack tight 
wound on 3 cores using RG400 to try on the fan that was a problem, or 
"bundle" the LMR400 winding that's there.  "Bundling" is easy (no, not 
that kind of Bundling) and as that choke is already there, it only 
requires lowering the antenna, which is easy with the rope and pulley 
arrangement.
With a substantial imbalance on 75, core heating would likely be a 
problem with only two, or three cores, so my concern is with the heating 
more than the common mode Z although common mode has proven to be a problem.

As I have the one antenna that is easily accessible and an AIM for 
measuring all parameters to a finer accuracy than needed, I plan on 
using this antenna as a test bed.  I definitely will try 3 and 5 cores 
with 1/8th in spacing (0.125") for cooling and tightly wound using RG-400.
I use a milling machine to cut slots for the cores in quarter inch thick 
Lexan with 1/8th inch spacing between slots.  The slots need to be 0.003 
to 0.005" over the half inch of the core thickness so they can be 
epoxied in place.

My weatherproofing gets complicated as I do not want the chokes enclosed 
because of cooling concerns.

I'm in the middle of changing the antenna system and have to depend on 
volunteers so change is slow. I have little stamina so I work for a 
while and rest for quite a while (rinse and repeat). Although I have 
that 6-pack, it currently has no power
Changing the fan on 80/75 to an 80/40 will be the simplest and quickest 
as I can do that change myself as long as my legs and back hold out.

It gets more complicated to explain than do.

Essentially, I will let the 75/80 fan down from the tower, then cut one 
leg on each side for 40 plus the insulator. As the 80 meter legs are a 
tad long, cutting the insulator off presents no problem with the loss in 
length. Actually, I need to shorten it even more. I will slide a PVC 
(1/2" tube) spacer over the 80 meter leg and slide it up to the end of 
the 40 meter leg. The spacer will float on the 80 meter leg. I will pull 
the poly rope through the spreader and tie it to the 40 meter end 
insulator. Knots on both sides of the spreader should keep it at the end 
of the 40 meter dipole. the rope goes through a pulley and back to the 
end of the 80 meter dipole. This keeps equal tension on both dipoles. 
Repeat on the other end.

I plan on pulling the loops of coax on the choke together to lower the 
frequency and raise the Z for the common mode RF.  At least that's what 
I hope to do.
It's been my experience that cutting the antenna to resonate at or a bit 
above the operating area will result in a lower voltage when operating 
at the lower end of the range than doing the reverse. On 160 an antenna 
resonant near 1.8 MHz is likely to cause the tuner to arc near 2.0Hz, 
while resonant near 2.0 MHz will not cause an arc when operating near 
1.8 MHz.  The lower Z of 160 limits tuner ranges substantially.

I like that f/b which is very important on 80 and 40.

With  the dipoles at 40, 110, 180, 255, and 320 degrees, the only 
limitation is at 180 degrees.  I have just enough room for a 160, half 
sloper that has one end at the tower. Under normal circumstances, I 
could move the bottom end another 10' South, but I now have a very 
unfriendly neighbor in that direction.  Normally my weakest radiated 
signal will be in that direction unless I'm trying for South America.

If I could keep the SWR down on 80, 8X would work at the legal limit, 
but 5 identical, remote switching devices adds complexity .
I have pulleys at the 97' level with the ends of the antennas about 10 
feet out. I could add a 10-15' 3" mast with the pulleys at 110 feet, 
using the 2" mast to the 7L C3i 6-meter Yagi at 115 feet, but would 
rather not.

I'd love to use RG400, but 100 feet is $230, or $2,000 for 1000 feet.  
I'd easily have over $1000 just in feed line. Were I still a project 
manager, I'd do it without a second thought, but a pension plus SS?  
Never thought I'd become economy minded.  OTOH I do have sources, I'd 
rather not use. Still I do have more than enough larger feed lines.  The 
weight of the choke and large feed line does not cause any noticeable 
sag in the 75/80 fan dipole, but it is under considerable tension.
The main drawback to five slopers fed with RG8 size coax is the visibility.

100 feet of RG-400 would  be more than enough to do the required chokes, 
but then connecting the larger feed lines and weatherproofing gets more 
complicated.  I might just as well make the chokes part of the feed 
line.  OTOH I'm certainly willing to try the tightly wound RG-400.

As I said in the previous post, I have a full 500' roll of BuryFlex and 
it appears there is plenty of LMR 400 in the generator shed.  Now if 
LMR-600 wasn't so heavy and didn't offer so much wind resistance...but 
it does and nothing will change that

73

Roger (K8RI)


On 6/15/2016 Wednesday 11:44 AM, Jim Thomson wrote:
> Date: Wed, 15 Jun 2016 02:10:21 -0400
> From: "Roger (K8RI) on TT"<K8RI-on-TowerTalk at tm.net>
> To:"towertalk at contesting.com"  <towertalk at contesting.com>
> Subject: [TowerTalk] OWAs or Fans?
>
> I have several center fed, half wave sloping dipoles for 80 and 40..
>
> I'm debating on putting up several 80 meter OWAs, or just making the the
> Fans for 80 and 40.  On 40, I can get away with RG8X at the legal limit,
> but not on 75/80.
>
> However I only operate below 3.7 and 7.200.The SWR remains relatively
> low, below 7.2 and easily compensated for with the AT5K-HP and the 8X
> handles it nicely. With 80, 3.7 to 3.5 is a different story, but the
> tuner still handles it nicely. OTOH the voltage might be a tad high for
> 8X with the high SWR. Still, using LMR 400, or the heavier BuryFlex(TM)
> will work and I "think" I can get enough turns through 7 cores, with the
> coax bundled instead of equally spaced turns around the cores, to tame
> the common mode RF.  If not, I can add a second choke as I have now.
>
> I'm leaning toward the fan instead of the OWA as I can get 5 slopers as
> in the antenna book for switchable directivity with a f/b ratio?  The
> ones to the NE and SE would be hidden by the trees.  Hopefully I can get
> the desired results with a single choke at each feed point.  That's a
> whopping 35 cores. A second 5 core takes it up to 60 cores as well as
> adding up to a lot of coax! For the 7 turn at roughly 21 feet the 5
> chokes add up to about  105 feet and about 80 feet  (I've not calculated
> that yet) is another 400 feet.  I probably shouldn't have given so much
> used LMR400 away.  I might still have 400 or 500 feet left.  I do have a
> new, 500 ft roll of BuryFlex, but I need a 100 feet of BuryFlex to get
> from the AV640 under the sod to the tower grounding plate and then to
> the 6-pack.
>
> Has anyone modeled one of these setups just using dipoles on any band?.
>

##  The older arrl books claim, with 5 x 1/2 wave slopers,  aprx  20 db FB..and  some
gain..like  3-4 db.   If you do the  CM choke right,  you wont require  35 x cores...nor  60. I talked to
a fellow who used the  5 x sloper system years ago, and  the 20 db FB was something to behold.
That was on 40M.   IMO, it was a superb ant.  Never heard one on 80m, but if the support was
high enough, it too should work good.   A bit of gain..and  20 db FB..with instant  72 deg switching,
would make most folks happy.... and no yagi / hd rotor to mess with.  Simple..and has merit.  No radials
to mess with.  Simple relay switching system.

##http://myantennas.com/wp/product/cmc-230-5k/     As  you can see from the graphs,
he attains 8200 ohms Z..and 8000 ohms RS  on 40m.... with just 4 x cores ( 2.4 OD, type 31)
and teflon coax.   ( 8.5 k Z  +  8K RS on 80m).  If you use a fan type dipole, sorta stagger tuned perhaps,
you should be able to achieve low swr.   Id suggest keeping the top ends away from the tower if at all possible.
Another possible option is to use larger gauge wire, like  at least 10 ga..and possibly 8 ga.
Either will result in a flatter  swr curve  on  80 /40m.

##  You should be able to do 5 x CM chokes  with no more than  20 x cores.  If you optimize the
CMs for just 40m,  you should be able to get the Z + RS even higher.

##  danny is currently experimenting with the larger .280 inch OD ( 14.5 gauge center conductor)
teflon coax.  Its slightly bigger than the .249 inch OD of RG-8X.   The typ  RG-303 /RG-400 coax
uses a 18 ga  center conductor.  RG-393 uses a 13 ga  center conductor.... but is aprx .392 inch OD.

##  This new coax hes playing with is mid way between the overkill  RG-393..and the smaller  RG-303 /400.
Since its not much bigger OD than  8X, it should be able to be wound on type  31 cores....and the peak V
rating is quire a bit more than 303/400...... but no where near what 393 is rated for.   A 100 kv hi pot tester
wont put a dent on 393 teflon coax..provided the dual silver plated braids are back from the ends.   If you can keep
the swr down, that alone will solve most of ur peak V issues.

Jim   VE7RF


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