Topband: Inverted L

[Joe Subich, W4TV]

> If you remove the slit PVC tubes you will be subjecting the 
> aluminum elements to stresses that they were not designed to 
> receive.  The original U-clamps will be of the wrong diameter 
> and will not fit around the elements correctly, compressing 
> from the top only. 

An even larger problem is that changing the elements of an 
F12 antenna from insulated mounting to grounded with the 
relatively large boom to element plate will detune the antenna. 
The element mounting plates will change the effective length 
of the elements and you will effectively have a different 
antenna.  How much that changes the performance of the antenna 
is hard to say but it's a big risk unless you know exactly 
what you're doing. 

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: topband-bounces at contesting.com 
> [mailto:topband-bounces at contesting.com] On Behalf Of Milt, N5IA
> Sent: Saturday, February 13, 2010 1:18 AM
> To: Guy Olinger K2AV; Gene Smar
> Cc: topband at contesting.com
> Subject: Re: Topband: Inverted L
> 
> 
> 
> ----- Original Message ----- 
> From: "Guy Olinger K2AV" <olinger at bellsouth.net>
> To: "Gene Smar" <ersmar at verizon.net>
> Cc: <topband at contesting.com>
> Sent: Friday, February 12, 2010 7:49 PM
> Subject: Re: Topband: Inverted L
> 
> 
> I had forgotten about the insulating of the parasitics on the 
> F12 stuff.  He would just have to remove the slit plastic 
> tubing (slit pvc water pipe) and tighten the element mount U 
> bolts directly on the parasitic element.  No extra flashing, 
> connections, etc needed.  If he doesn't remove the slit tube 
> and loads up the tower, QRO RF will flash over the plastic any way.
> 
> I have a C31XR and will have to do the same thing if I load 
> my 76' tower.
> --------------------------------------------------------------
> -----------------------------------------
> --------------------------------------------------------------
> -----------------------------------------
> 
> NOT SO!!!
> 
> I have constructed this type of installation for other hams.  
> One is an 80' 
> length of Rohn 25 with the base elevated 40' AGL.
> 
> The 80' is topped out with an F-12 model C4XL which puts it 
> at 120' AGL. 
> The 160 vertical is series fed and has 2 dozen 1/4 WL 
> elevated radials 
> sloping down to an average of 10' AGL at their ends.
> 
> The 160 Meter energy is choked off the feed lines and rotor 
> cable for the 
> C4XL at the insulated base by coiling 30 turns of 6" diameter 
> in each line.
> 
> By hanging a 75/80 Meter double extended Zepp off the end of 
> a wooden Xarm 
> at the 100' level, this single structure is a very high 
> performance antenna 
> system on all bands from 1.8 mHz to 29.7 mHz with the exception of 30 
> Meters.
> 
> My point in relating this is that the parasitic elements of 
> the C4XL were 
> not modified and grounded.  There has never been any flash 
> over at full 
> legal limits on Topband through HUNDREDS of hours of 
> contesting and casual 
> operation.
> 
> If you remove the slit PVC tubes you will be subjecting the aluminum 
> elements to stresses that they were not designed to receive.  
> The original 
> U-clamps will be of the wrong diameter and will not fit 
> around the elements 
> correctly, compressing from the top only.  If you change to 
> smaller size 
> U-clamps to fit the element you will have to redrill the 
> element mounting 
> plates.
> 
> Too much hassle and posssibility of something going wrong, for a 
> modification that does NOT need to be done IMHO and my real 
> life experience.
> 
> YMMV.  Milt, N5IA
> 
> 
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK