[TowerTalk] Building a W6NL Moxon 40 Meter Beam.

[David Gilbert]

There isn't really a "shielding" effect, at least not in theory.  If the 
driven element was entirely surrounded by an equal length tube the RF 
would merely be coupled to the tube and it would become the radiator.  
The RF does not get "grounded" any more than if the center of the driven 
element was itself bonded to the boom.  So yes ... the channel just 
becomes an equivalent part of the center of the driven element (assuming 
electrical balance, of course).  There might be some tuning/detuning 
effects of the combination of the channel and the element inside it, but 
I would think they would be minor and you'll never even know they are 
there anyway because all you'll ever see is the net effect of everything 
that's up there. The only way any of those effects would be deleterious 
is if the capacitive coupling of the driven element to the channel has 
significant loss ... and I don't think that would be the case since the 
RF voltage at that point (and therefore the capacitive current though 
the dielectric) is very low.

You're going to love the antenna if you can keep from losing sleep about 
it in the meantime.   ;)

73,
Dave   AB7E




On 8/3/2014 3:27 PM, L L bahr wrote:
> Bud and Dave Gilbert,
>
> OK, I understand what you are saying, Bud.  But what about the shielding effect?  Taking this to an extreme, I could encapsulate the entire driven element into a hypothetical long grounded tube.  No RF could get out of it due to it's 100% shielding. It would be 100% ineffective.  Yes, the aluminum channel is only 2 foot long, but it's 2 foot is shielding 3 sides of the driven element right at where the maximum radiation is taking place. The driven element only has the top open for 2 foot of its area where most of the radiation is taking place.  How can this shielding on three sides not be affecting the amount of RF getting into the ether?
>
> Dave,  I see you wrote also.  Maybe you are starting to get through to me.  You are saying the boom which is bolted to the channel is not at RF ground and is thus not shielding the driven element as Bud is pointing out the channel just becomes part of the center part of the driven element.
>
> Is this what you guys are trying to beat into my skull?  (I'm starting to feel better now!)
>
> Lee, w0vt
>
> ----- Original Message -----
> From: "W2RU - Bud Hippisley" <W2RU at frontiernet.net>
> To: "L L bahr" <pulsarxp at embarqmail.com>
> Cc: towertalk at contesting.com
> Sent: Sunday, August 3, 2014 5:08:10 PM
> Subject: Re: Building a W6NL Moxon 40 Meter Beam.
>
>
> On Aug 3, 2014, at 5:34 02PM, L L bahr <pulsarxp at 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 at verizon.net" <scottw3tx at verizon.net>
>> To: "L L bahr" <pulsarxp at embarqmail.com>
>> Cc: "David Gilbert" <xdavid at cis-broadband.com>, towertalk at 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
>> _______________________________________________
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