[TowerTalk] add a 3rd el to 40-2cd

Jim Thomson jim.thom at telus.net
Tue Mar 13 23:44:36 EDT 2018


Date: Tue, 13 Mar 2018 12:54:18 -0400
From: k7lxc at aol.com
To: bmarx at bellsouth.ne, towertalk at contesting.com
Subject: Re: [TowerTalk] add a 3rd el to 40-2cd

?
> ?With one antenna at 72 ft why not just add new antenna lower on the tower>?
?
? ? One of my all time favorite ideas. Stacked would give you similar performance to a full sized 3L plus you get angle and directional diversity. And you don't need expensive rotators! Like my old buddy Mark, AA6DX sez: "Worky, worky!"
?
Cheers,
Steve ? ? K7LXC


##  utter waste of time and effort.  You would also have to be able to rotate the lower yagi... =  huge  $$$  for  2nd yagi and lower rotating assy.  
With the top 40m yagi at  a paltry 72 ft, it will barely work as is.  Now you want to place a 2nd 40m yagi at 36 ft ? 

##  Think about this for a minute.   Put a 20m yagi at 36 ft..and a 2nd 20m yagi at 18 ft... what do you think will be the results ?  

##  here on the west coast, even a 20m yagi at  72 ft...  with a 2nd  20m yagi at  36 ft is essentially a wasted effort..esp on flat level ground. 

##  The main problem with the 402-CD is the loading coils.  http://www.qsl.net/ve6wz/intro.htm  scroll down til u see the info on the  XM-240. 
http://www.qsl.net/ve6wz/CC_coil.html   12 gauge magnet wire, wound butt tight,  on a .75 inch solid fiberglass rod  is as useless as it gets for a 
40m loading coil.  Q is through the floor..and coil losses are sky high.   No wonder they end up with a 50 ohm input Z..with NO matching  device used. 
The XM-240 is a direct fed  2 el  40m shortened yagi.   When was the last time you saw a 2 el yagi that did not have a way less than  50 ohm input Z ? 
The coil losses are sky high on the inferior  designed coils, such that the real feedpoint Z  plus coil losses = 50 ohms..... = fubar.

##  .1875 tubing, in either  al or cur  would increase the 40m coil Q from  81  up to  767 / 1200 ohms.   The .1875 al tubing retrofit coil that Steve cooked
up weighs  1 oz more than the  CC junk it replaced. 

##  a  3 el version of the CC  shorty 40, using the same oem 12 gauge magnet wire would just increase losses even more, rendering the  proposed 3el yagi  another wasted effort.
Now if the proposed 3 el 40m shortened yagi was constructed using low loss  al or cu coils, the 40m yagi would then function correctly..with out all the losses from the 6 x coils. 

##  The oem capacity hats are questionable at best.  They are way too small...and way too close to the coils. 

##  Any 40m yagi requires a good  CM choke, like depicted on K9YCs cookbook.   6 turns of  213 U  wound onto 5 x type  31 cores  will provide for 6500 ohms of Z on 40m. 
2 of them in series will provide for  13,000 ohms of Z.    Constructed like that, the 40m yagi will handle all the power you can throw at it..esp if  RG-393 teflon coax is used and 
2 x CM chokes are used in series =  bomb proof.  

Jim  VE7RF



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