[NLRS] Yagi Spacing Q.. - MORE questions

Thu Oct 23 15:27:40 EDT 2008

Thanks for the info Donn.  I'm learning a lot more of this reading websites 
and from your guy's comments.  I have to admit i usually just mounted stuff 
like this however the manufacturer recommended, but its also good to know 
the technical aspects behind it.

I should have this done in the next month, fingers crossed, it depends on 
the weather, available time of those that can help put it up, and a little 
money to finish buying a few things.  The money is the biggest stumbling 
block for me these days though!  Nice to see copper prices coming down, i 
hope that trickles down to the cost of coax.

73,

Scott

----- Original Message ----- 
From: "Donn Baker - WA2VOI/0" <wa2voi at mninter.net>
To: "KB0NLY" <kb0nly at mchsi.com>; "NLRS" <nlrs at mailman.qth.net>
Sent: Thursday, October 23, 2008 2:17 PM
Subject: Re: [NLRS] Yagi Spacing Q.. - MORE questions

> 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
>>>>
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>>>
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