[VHFcontesting] stacked yagi's

Tue Apr 13 14:16:21 PDT 2010

FIRST: As regards the impedance-transforming use of 75-ohm coax in a coaxial 
power divider, David is right. You need to use ODD multiples (e.g., x1, x3, 
x5, etc.) of a QUARTER wavelength. That's an ELECTRICAL quarter-wavelength, 
derived by multiplying the physical quarter wavelength by the velocity 
factor of the coax being used -- probably RG-59/U or RG-11/U. Or maybe 
RG-6/U (cable TV stuff). Whatever, it must be 75-ohm coax. Look up the 
velocity factor in the data sheet for that specific kind of 75-ohm coax. 
They are NOT all the same. Using 75-ohm coax in this manner creates a 
transformer, matching your 50-ohm feedline to your two 50-ohm yagis through 
the coaxial T connector.

Always adjust each yagi's matching device (gamma or whatever) INDIVIDUALLY 
using 50-ohm line and ideally a complex impedance analyzer like the MFJ-259B 
BEFORE stacking the yagis. Your two yagis should each show 1:1 SWR at the 
feedpoint BEFORE you try to stack them. DO NOT try to adjust matching 
devices on yagis that are already stacked and wired through a T connector.

Whether you can use two identical 1/4-wavelength pieces of 75-ohm coax for 
your divider/transformer will depend on how far apart the antennas are 
spaced. If they are too far apart for two 1/4-wavelength pieces to reach 
(probably the case), the simplest thing to do, AND THE LEAST PRONE TO ERROR, 
is to simply use the next odd multiple of length for BOTH pieces.

You CAN use one 1/4-wavelength piece and one 3/4-wavelength piece, but then 
you have to think about correcting for the 180-degree phase shift you've 
just introduced. THIS IS IMPORTANT. If you DO NOT want to have to worry 
about phasing corrections required by the use of different lengths of 75-ohm 
coax, simply make both pieces of 75-ohm coax exactly the same odd number of 
electrical quarter wavelengths long -- i.e., both 1/4-wavelength, both 
3/4-wavelength, both 5/4-wavelength, etc. That's the safe and sure method.

SECOND: When using a 50-ohm power divider with all 50-ohm coax (no 75-ohm 
coax transformer), Paul is right. It doesn't matter how long the two pieces 
going from the power divider to the two yagis are, as long as they are the 
same kind of coax (ideally, cut from the same roll so the velocity factors 
are identical) and are exactly the same length. Just as with the 75-ohm 
coaxial transformer, any difference in ELECTRICAL length will introduce an 
unwanted phase shift between the antennas.

So to summarize: To keep the antennas in phase, place them so that their 
matching components are on the same sides (for example, both on top, both to 
the left) and keep the lengths of the pieces of coax running to each antenna 
from the T exactly the same.

All of the above can be found in any ARRL Antenna Book, and everyone should 
be encouraged to look it up there and check. You will also find detailed 
information about how to compensate for the phase difference introduced by 
using unequal lengths of coax between the divider and the yagis (if you're 
absolutely bent on doing it that way).

Bill W5WVO

--------------------------------------------------
From: "David McKenzie" <k1fsy at vhfwiki.com>
Sent: Tuesday, April 13, 2010 9:32 AM
To: "Paul Kiesel" <k7cw at yahoo.com>
Cc: <vhfcontesting at contesting.com>
Subject: Re: [VHFcontesting] stacked yagi's

> Everything I've read about phasing two 50 ohm yagis with 75 ohm cable
> stipulates equal, odd multiples of a quarter wavelength at the desired
> frequency. Both feed points should face the same direction; if you are
> stacking vertically, then both are on the bottom; horizontally, both on 
> the
> left. I have two 2M9FMs horizontally stacked using 9/4 wavelengths of RG59
> coax to achieve the proper match and make the distance to the mast in the
> center.
>
> When stacking with 50 ohm coax I have seen even half-wave multiples used.
>
> http://www.scribd.com/doc/8511618/Building-Phasing-Harnesses
> .
>
> On Sun, Apr 11, 2010 at 1:56 PM, Paul Kiesel <k7cw at yahoo.com> wrote:
>
>> If you use a power divider, use two equal lengths (it doesn't matter how
>> long they are) of 50 ohm coax, from the power divider ports to each yagi.
>> The impedance transformation is going on in the power divider, so  you
>> already have 50 ohms available at each of the power divider's ports.
>>
>> If you elect to use coax, use 75 ohm coax so you get the proper impedance
>> transformation. You must use exact multiples of an electrical half
>> wavelength. You will want to use as little coax as possible to reduce 
>> your
>> losses in the coax, so if you elect to make both lengths of coax an EVEN
>> number of half wavelengths long, then you will orient the feeds of the
>> antennas so that they are both on the same side, symmetrical with each
>> other, if you will. But, if you decide to have one length of coax be an 
>> EVEN
>> number of half wavelengths long and the other length of coax an ODD 
>> number
>> of half wavelengths long, then you will need to reverse the feed on the
>> second yagi so that it is reverse from the first yagi. This is because 
>> the
>> extra half wavelength of coax that you have going to one of the yagis 
>> will
>> result in a 180 degree shift in phase. So, in order to get both antennas 
>> in
>> phase with each other, you will need to reverse the feed on one of them 
>> by
>> flipping the
>>  antenna upside-down or by mechanically re-arranging the feed.
>>
>> 73,
>> Paul, K7CW
>>
>>
>>
>>
>> ________________________________
>> From: Jon Casamajor <k6el at comcast.net>
>> To: vhfcontesting at contesting.com
>> Sent: Sat, April 10, 2010 2:09:05 PM
>> Subject: Re: [VHFcontesting] stacked yagi's
>>
>> Hi Paul,
>>
>> I am a little confused on what you are doing I guess.   I have two M²
>> 420-50-11 Yagis stacked vertically, that is one above the other.  I am
>> using
>> a standard M² splitter and stacking harness and the distance between the
>> Yagis is 28” and the phasing lines are identical in length however I’m 
>> not
>> entirely sure of their length as they are living at about 100’ now.  J
>>
>> You said… “(note that I might switch units without warning)”, so I may 
>> not
>> be seeing something here.  If both phasing lines are the same length
>> wouldn’t they be fed in phase?  My little stack is on my QRZ page in case
>> I’ve confused you in return.  The little stack works quite well but I’m
>> planning to replace it soon with a 4329WL as the stack is a little sharp
>> and
>> I need a bit more ERP.
>>
>>
>>
>>
>>
>> 73, Jon
>>
>> K6EL
>>
>> -----Original Message-----
>>
>>
>>
>>
>>
>> ------------------------------
>>
>>
>>
>> Message: 2
>>
>> Date: Sat, 10 Apr 2010 03:07:25 +0000 (UTC)
>>
>> From: Paul Decker <kg7hf at comcast.net>
>>
>> Subject: [VHFcontesting] stacked yagi's
>>
>> To: vhfcontesting at contesting.com
>>
>> Message-ID:
>>
>>
>> <
>> 1210296233.8429611270868845217.JavaMail.root at sz0009a.westchester.pa.mail.co
>> mcast.net>
>>
>>
>>
>> Content-Type: text/plain; charset=utf-8
>>
>>
>>
>>
>>
>>
>>
>> Hey group,
>>
>>
>>
>>
>>
>>
>>
>> I'm in the process of stacking two 11 element 70cm yagi's.? I've modeled 
>> it
>> in eznec and I think the optimum distance is about 42".? (note that I 
>> might
>> switch units without warning).
>>
>>
>>
>>
>>
>>
>>
>> Once I have the two 50 yagi's stacked, my model says I need to have one 
>> 180
>> out of phase, does this seem right?
>>
>>
>>
>>
>>
>>
>>
>> If that is the case, then I need different lengths of coax to feed each
>> antenna.
>>
>>
>>
>>
>>
>>
>>
>> Should I use a 50 ohm power divider and feed each antenna with 1.25w and
>> 1.75w 50 coax, or should I feed with 1.25 and 1.75 75 Ohm coax and simply
>> use a T connecter.? Is there an advantage of one over the other?
>>
>>
>>
>>
>>
>>
>>
>> thanks for any help, 73,
>>
>>
>>
>> Paul (KG7HF)
>>
>>
>>
>>
>>
>> ------------------------------
>>
>>
>>
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>> End of VHFcontesting Digest, Vol 88, Issue 9
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