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Re: [TowerTalk] Motor Driven vac used in 40M DE?

To: "Richard \(Rick\) Karlquist" <>
Subject: Re: [TowerTalk] Motor Driven vac used in 40M DE?
From: "Jim Thomson" <>
Date: Wed, 30 Dec 2009 00:50:42 -0800
List-post: <">>
> I did some antenna range tests with my MonstIR, varying the driven element 
> length while leaving the director and reflector fixed.
> You can improve the SWR bandwidth doing this, but the gain and
> F/B drop off as you get away from the optimum frequency
> to which the parasitic elements are tuned.  You really need to
> tune all 3 elements as is done in the MonstIR.
> Rick N6RK

### I was under the assumption, reading the F-12 specs... that when the DE 
was tuned for 7070............ the yagi was good for  7000-7280 khz .     It 
appears that the swr rises slowly on the high side of resonance.

## Now it seems that the 2:1  swr may well be  280 khz.... but the F/B  and 
possibly the gain is not any where near  280 khz.      If  I recall.... most 
3 -el yagi's   when tweaked correctly, don't even have the freq  for max 
gain  coinciding with the freq  for  best  F/B

##  300 khz  wide on 40m... is like  600 khz wide on 20m....... and this 
340N  uses  85%  of  full size els... ranging from aprx  52'  dir.... to the 
59'  ref.     That's a tall order.    Plan B  might be to include relay 
box's  and coils at both the REF  and the DIR..... and  allow for 2-8  x 
segments on each parasitic ele.    I was trying to avoid that scenario.. 
like the plague.      Looks like I will have to go that route, in order to 
get  half decent  fb/gain  across the band.

## I also have the  F-12  40m relay box... which consists   of  2 x 12 ga 
thnn coils... and a dpst deltrol  relay  [ identical to a P+B 12 vdc relay] 
The contacts of the relay  have gone int.. brand new, out of the box !    I 
applied.. "cool amp"  which is  silver plating paste to the contacts.. and 
presto.. back to zero ohms again.   I have used that stuff on buss bars, 
contactor's and all sorts of stuff.     Then a tiny bit of... "conducto 
lube" was also applied .  It's just grnd up silver powder.. in grease..... 
made for sliding contacts.      I also replaced the 12 ga thnn  coils  with 
8 ga polyimide  magnet wire, etc.   The real fix is to use vac relays.. and 
be done with it.

### I can easily get at the DE... plus the tower that's being installed this 
spring is a HDX-689... so the 40m boom will only be 25'  above grnd.. with 
tower nested.    The boom, when tilted.. will allow the dir/ref  to touch my 
roof... for access.

###  I have a few of these motor driven caps.... and will be using an 
identical cap  on my old  EF-180B    f-12   80m rotary dipole.     I'm not 
happy with my  F-12   3 x relay/6x coil  / 8 x segment  switch box.   It 
needs  8 x coils  and  4 x relays to do the job right  [ 16 x segments] 
If spst vac relays  are used.... 8 x relay's required.     I came to the 
conclusion that one motor driven cap makes more sense, that 8 x coils and 8 
x vac relays !

### the project started  with the 80m  F-12 dipole... and since the motor 
driven cap was to be used... I thought, why not use the other cap   on the 
340N  DE !      Now it appears that I'm in a  40m  dilemma.... and that 
I'll have to add coils and relays to the dir /ref..... and a motor driven 
cap to the DE.   I need to be able to cover the entire band.    The motor 
cap is 9 lbs.   Adding one to a DE is no big deal.... but I'm not abt to add 
caps to the dir/ref.. due to weight constraints.

##  I don't have the room  for a monstIR 40m yagi.    My buddy had one up 
135'.... which promptly destroyed his new  OR-2800 rotor in one week flat.

##  I was going to try and keep the 40m yagi .. "simple"..... but  I see 
there is no free lunch... so this will be yet another gong show.

Later............. Jim  VE7RF 


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