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@frontiernet.net>
To: "L L bahr" <pulsarxp@embarqmail.com>
Cc: towertalk@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@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
> _______________________________________________
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