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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