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

[Grant Saviers]

The W6NL Moxon design uses schedule 80 PVC pipe for the driven element 
insulation.  F12 at one time did use ABS pipe since that was on my 
EF180C.  ABS drain pipe is a bad idea since the core is foamed and 
compresses easily over time, so the element clamps were loose on the 
EF180C.

The tolerances for the fit of the Moxon insulator are too tight at the 
extremes allowed by the aluminum od and PVC pipe id specifications.  
Heating the PVC in some boiling hot water solves that problem since the 
PVC expansion coefficient is sufficient to increase the id enough.  One 
antenna we built went together easily, the other needed the PVC heated up.

Grant KZ1W


On 8/5/2014 8:36 AM, Pete Smith N4ZR wrote:
> Force 12, for many years, used ordinary gray PVC tubing, split 
> lengthwise, to insulate its elements from the boom and any metallic 
> mounting hardware.  The element mounts are aluminum sheet hat 
> sections, pop-riveted to the boom and u-bolted to the element/pvc 
> sandwich.
>
> 73, Pete N4ZR
> Check out the Reverse Beacon Network at
> http://reversebeacon.net,
> blog at reversebeacon.blogspot.com.
> For spots, please go to your favorite
> ARC V6 or VE7CC DX cluster node.
>
> On 8/4/2014 5:42 PM, Georgens, Tom wrote:
>> I have built two of these antennas and I am very pleased.  While I do 
>> not have an antenna range to measure pattern, they seem to be very 
>> solid performers.  SWR is below 1.3:1 across the entire band.  The 
>> prime motivation was reducing maintenance.  The 403BA needed more 
>> maintenance interventions than the other 5 yagis combined (probably 
>> times 2).  So far so good.
>>
>> One was built starting with a 402CD and the other from scratch. I 
>> found cleaning the old aluminum tedious and probably more time 
>> consuming that starting with new tubing.  All cutting was done on a 
>> table saw with a cheap abrasive blade and all drilling was done with 
>> a home depot drill guide.
>>
>> Since it is not a kit, some ingenuity is necessary on the element 
>> mounting and, should you go that way, the element supports.  I used 
>> channel and the DXengineering blocks to mount the elements.
>>
>> One thing you should consider is how to get the antenna on the 
>> tower.  A crane or a tram is the best bet.  If you are going to twist 
>> it through guy wires, or assemble at the top of the tower, the T-bars 
>> make this approach very complicated.  I did the twisting method and 
>> we could not get it done without dropping a guy at each level.
>>
>> Good luck
>>
>> 73, Tom W2SC 8P5A
>>
>> -----Original Message-----
>> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf 
>> Of L L bahr
>> Sent: Sunday, August 3, 2014 1:07 PM
>> To: TowerTalk
>> Subject: [TowerTalk] Building a W6NL Moxon 40 Meter Beam.
>>
>> As mentioned on another post and subject,  I am going to build a 40 
>> meter W6NL Moxon Beam. The center of the Director is split and is not 
>> at ground potential. I see a lot of builders have used a two foot 
>> long aluminum channel to mount the Director and Reflector to the 
>> boom. They use a plastic or fiberglass sleeve to isolate the director 
>> from the grounded Boom. I can see this would make for a very strong 
>> support.
>>
>> 1.  All this to me means, 2 foot of the radiating part of the 
>> Director is close coupled to ground.  I'd think this is Bad!! Seems 
>> to me the use of a Delrin spacer rod or fiberglass sleeve with just 
>> the use of U-bolts without the 2 foot of aluminum channel would 
>> dramatically reduce the Director coupling area to the grounded 
>> aluminum channel.
>>
>> 2.  Also, the feed point on all the versions I have looked at couple 
>> an RF choke right to the center point area of the Director.  When I 
>> built a couple plain Yagi antennas, I used a Gamma Match or T match 
>> thus tapping the Director element at a 50 ohm point rather then at 
>> say at a 15 ohm point or so right at the center of the director.  Why 
>> is this not done on the W6NL building examples?  Is it because they 
>> think and hope the RF balun choke is going to keep the mismatch SWR 
>> in check?  Can anyone explain this to me? I'm not sure, but I suspect 
>> the impedance right at the center of the Director is not 50 ohms or 
>> even close to being 50 ohms.
>>
>> Anyway,  I thought I would throw these two concerns out for you 
>> experts to comment upon. Am I thinking clearly and correctly or am I 
>> wrong somewhere?
>>
>> Lee, w0vt
>> Houston
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