[NLRS] Yagi Spacing Q.. - MORE questions

Thu Oct 23 15:17:39 EDT 2008

Hi Guys.
My 2 cents worth:
Jim's (K0MHC) comments are generally correct.  However, "through" the capture area, not necessarily "outside" of the 
capture area is maybe more correct.  And, yes, anything behind the reflector is outside the capture area.  Mounting an 
end-mount vertical antenna on a vertical metal mast is OK.  Mounting a vetical antenna with a vertical metal mast, say 
at the balance point of the boom, will cost you 10 dB or more of gain, and mess up the match and pattern as well.  Its 
like putting a short-circuit in the field of the antenna.

Likewise, using a vertical metal mast that is "large compared to the wavelength" to mount a horizontally polarized 
antenna at the balance point will also cost you some gain, etc.; more if it pass completely through the capture area. 
Ex: 1-1/4" mast for 2m is OK; that same mast for 1296MHz is pushing it; don't even think about it for 3456 MHz if it 
passes through the antenna.

There are special ways of doing the vertical metal mast for a vertical antenna, but they're generally way to complicated 
for the gain (no pun intended).  The most common is to mount the antenna by the (vertical) driven element, using a 
1/4-wave choke below to isolate the driven element from the support mast, RF-wise.  This complicates the feed system, 
too, but sometimes its worth it.

Good luck.

73 Donn
WA2VOI/0

----- Original Message ----- 
From: "KB0NLY" <kb0nly at mchsi.com>
To: "NLRS" <nlrs at mailman.qth.net>
Sent: Thursday, October 23, 2008 6:42 PM
Subject: Re: [NLRS] Yagi Spacing Q.. - MORE questions

>
>
> Yes, your correct, they are end mounted.  And I would think there would be less interaction problems with the mast as 
> well.  In my case they will be mounted on a crossboom to use less vertical space on the towers mast.  So there will be 
> a mast perpendicular to the elements but only on one side, well a few inches past the clamp on the other side.
>
> Never tried one of those dualband yagi's but often wondered the very same thing myself.  I also wondered how they made 
> them work with a single input, looks like two specific lengths of coax to a "T" and then you connect the feedline to 
> that.
>
> I collected these Yagi's over time, the 124WB was a cheap eBay find, brand new in the box, and the A449-6S was a used 
> one I got for next to nothing. So I figured I would try them for local FM use, maybe get into a few distant repeaters 
> a bit better, rather than buy a new dual band one for the same purpose.
>
> 73,
>
> Scott
>
>
>
>
>>
>> Not to questions Jim's response...BUT, and _particular_ to Scott's
>> specific antennas.
>>
>> Since the 124WB and A449-6S are end mounted designs...wouldn't there
>> be 'less' worry about such interaction with the mast and/or coax?
>>
>> In Cushcraft (Laird?) pictures (with the antennas by themselves), they
>> are shown with a mast (presumably metal) only inches behind the
>> reflector.
>>
>> ALSO, Cushcraft (Laird?) makes (made?) the A2706S and A27010S Dual
>> Band Yagis where elements are on the SAME boom, essentially 'minimal'
>> spacing between the two antennas.
>>
>> So I guess my questions would be...
>>
>> 1. IS a "rear" mast out of the 'capture area' for the antenna or not?
>>
>> 2. Has anyone done an evalation of those Dual Band Yagis? How DO they
>> perform with, basically, TWO antennas on the same boom? Probably
>> seeking a "more thorough" assessment than "just" max. forward gain as
>> is typically done at conference antenna ranges, e.g. 'cleanliness' of
>> pattern, any "odd" nulls in the forward direction, and the like.
>>
>> 73, JK
>> ----- Original Message ----- 
>> From: "Jim Froemke" <jim.k0mhc at earthlink.net>
>> To: "'KB0NLY'" <kb0nly at mchsi.com>; "'NLRS'" <nlrs at mailman.qth.net>
>> Sent: Thursday, October 23, 2008 11:00
>> Subject: RE: [NLRS] Yagi Spacing Q.. - response
>>
>>
>>>
>>>
>>> Hi Scott
>>>
>>> For your application (vertical polarization) a bigger issue is the
>>> proximity of each antenna to the vertical metal mast. For best
>>> performance
>>> you'll want to keep the vertical metal mast OUTSIDE of the capture
>>> areas of
>>> both antennas. This means your horizontal spacing along your
>>> horizontal mast
>>> between each antenna and the vertical mast should be AT LEAST 1/2
>>> the
>>> diameter of each antenna's capture area.
>>>
>>> You could use a non conductive vertical mast (if it's strong enough)
>>> but when your coax makes a 90 degree angle to descend down the
>>> vertical mast
>>> it also becomes a vertical conductor located on the vertical mast
>>> between
>>> the two antennas so you'll still have this problem.
>>>
>>> 73, Jim
>>> K0MHC
>>>
>>> _______________________________________________
>>> NLRS mailing list
>>> NLRS at mailman.qth.net
>>> http://mailman.qth.net/mailman/listinfo/nlrs
>>>
>>
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