Towertalk
[Top] [All Lists]

Re: [TowerTalk] Building a W6NL Moxon 40 Meter Beam.

To: L L bahr <pulsarxp@embarqmail.com>
Subject: Re: [TowerTalk] Building a W6NL Moxon 40 Meter Beam.
From: W2RU - Bud Hippisley <W2RU@frontiernet.net>
Date: Sun, 3 Aug 2014 18:08:10 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Aug 3, 2014, at 5:34 02PM, L L bahr <pulsarxp@embarqmail.com> wrote:

> Why won't the 2 foot long channel not be reducing efficiency on the Director 
> or will it indeed not induce inefficiency?

Hi, Lee —

If you accept that the aluminum elements of a typical Yagi have very little 
loss due to resistance, then perhaps you can accept the possibility that a 
2-foot long piece of aluminum channel doesn’t have much loss due to resistance, 
either.  So how is the channel reducing efficiency?  Reduced efficiency implies 
greater losses somewhere — usually in the use of materials having excessive 
resistance or by placing the antenna in close proximity to a LOSSY surface, 
such as common soil — which DOES exhibit a resistive loss.  But the 2-foot long 
aluminum channel isn’t introducing those kinds of losses.  For all practical 
purposes, it isn’t introducing any losses at all.

Instead, perhaps you should think of the 2-foot length of aluminum channel as a 
“parasitic element”.  Now, it’s true that parasitic elements can distort the 
radiation pattern of other elements.  But the dimensions of the aluminum 
channel are so short that it has little or no effect on the radiated pattern at 
7 MHz.  Have you ever seen multi-band Yagi antennas with elements for different 
bands interlaced with each other?  Have you looked at the design of the 3-band 
Yagi used by all the WRTC 2014 competitors last month?

Another way to think about the aluminum channel is to start with a driven 
element with a very, very thick-walled aluminum stock near the center of the 
driven element.  Now use a special (fictitious) saw to “shave off” part of that 
element on the first 2 feet of its underside.  Next, move the shaved-off part 2 
inches away from the remaining element stock, keeping the two metal rods 
parallel.  So what?!  Yes, there’s coupling between the driven element and this 
newly formed length of aluminum but — again — it’s too short and too close to 
have any appreciable effect on the radiation pattern from the driven element.  
At the very worst moving this aluminum stock from the driven element to the 
channel may have changed the taper schedule for the element but odds are high 
the designer has already examined this with an antenna modeling program and 
made whatever adjustments s/he felt necessary to optimize the performance of 
this particular Moxon implementation.

In summary:  Placing an excellent conductor near a resonant or near-resonant 
element of an antenna does not create loss or reduce efficiency.  If this added 
“parasite” has appropriate dimensions and position with respect to the original 
element, it MAY cause distortion of the original element’s radiation pattern, 
but it does NOT cause reduced efficiency.  At 7 MHz, a 2-foot length of 
aluminum channel 2 inches from the driven element does neither.

Bud, W2RU



> ficiency on a car body.)  I need to get rid of my fear.  Just seems to me the 
> channel mounting scheme is mechanically strong and is no problem for the 
> grounded Reflector, but it is not a good idea electrically at the Director 
> for efficiency. Somebody explain to me why my fear is not correct. Where is 
> my thinking flawed?  (I hope I am not upsetting the group with my 
> persistence.)    
> 
> Lee, w0vt 
> 
> ----- Original Message -----
> From: "ScottW3TX@verizon.net" <scottw3tx@verizon.net>
> To: "L L bahr" <pulsarxp@embarqmail.com>
> Cc: "David Gilbert" <xdavid@cis-broadband.com>, towertalk@contesting.com
> Sent: Sunday, August 3, 2014 4:14:17 PM
> Subject: Re: [TowerTalk] Building a W6NL Moxon 40 Meter Beam.
> 
> Ive used this antenna at K3LR. It works very, very well!  Dont change 
> anything and you will be very happy :)
> 
> Best regards,
> Scott W3TX
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk

_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>