Mark, Jim.... Thanks for the comments.
The feedpoints of both SteppIR antennas are identical as far as the unun and
overall DE construction are concerned, but the element spacing does vary
between the DB36 and 4E. I will need to play with the combination a little
more over time and try to get an overall feel as to how it plays. Like you
suggest, I can always easily add in various length stubs and experiment, since
the stack box is at the base of the tower.
Eric
K2CB
----- Original Message -----
From: towertalk-request@contesting.com
To: towertalk@contesting.com
Sent: Sunday, September 1, 2013 1:48:04 PM
Subject: TowerTalk Digest, Vol 129, Issue 3
Message: 3
Date: Sun, 1 Sep 2013 12:06:07 -0500
From: "Mark Beckwith" <r-emails@n5ot.com>
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Method of calculating phase delay variation
when stacking two dissimilar yagi antennas?
Message-ID: <25F253F203EF4ED696ADEBA7A31D32A4@NEVUSCEO>
Content-Type: text/plain; format=flowed; charset="utf-8";
reply-type=original
> So my question at this moment is - which is the
> proper method for me to calculate the proper
> delay line length to properly phase the two
> dissimilar SteppIR yagis?
This is not meant to be a snarky or otherwise "cute" response. Just more
like a reality check. The answer is:
Feed them with equal length feedlines and see how they play. If you like
the way they work, then you're done. I suspect you will like the way that
they work and I also suspect changing one feedline length by as much as 8
feet would not affect your overall conclusion.
Neither the WX0B formula or the WN9O formula accounts for imbalance caused
by the dissimilar feedpoints (even if the feedpoints were identical in every
other respect, one is still closer to the ground than the other) - meaning
there are undoubtedly unknowns in the mix which from the seat-of-my-pants
means it may largely be an academic question, likely trumped by actual
implementation.
Get on the air and work guys. I bet you work more guys with two antennas at
you disposal than just one antenna, and I bet when you do
top-bottom-bip-bop* tests you will find some times when some stations are
lounder under some of those four conditions. I suspect this would be the
same conclusion whether your coaxes are the same length or different lengths
by 4 feet or 8 feet.
This is what I would do anyway, and what I have found to be true in the past
when stacking dissimilar tribanders in real life?.
73 - Mark, N5OT
* Top antenna alone, bottom antenna alone, both antennas in phase, both
antennas out of phase
------------------------------
Message: 6
Date: Sun, 01 Sep 2013 10:22:23 -0700
From: Jim Lux <jimlux@earthlink.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Method of calculating phase delay variation
when stacking two dissimilar yagi antennas?
Message-ID: <522377CF.1040401@earthlink.net>
Content-Type: text/plain; charset=UTF-8; format=flowed
On 9/1/13 10:06 AM, Mark Beckwith wrote:
>> So my question at this moment is - which is the
>> proper method for me to calculate the proper
>> delay line length to properly phase the two
>> dissimilar SteppIR yagis?
The calculations are decidedly non trivial. The two antennas will
interact, small changes in the lengths of the elements (SteppIR, after
all) or their relative orientation (wind) will change the feed point
impedances, particularly the reactive component. That won't change the
"match" (in terms of VSWR) but will change the relative phasing.
>
>
> This is not meant to be a snarky or otherwise "cute" response. Just
> more like a reality check. The answer is:
>
> Feed them with equal length feedlines and see how they play. If you
> like the way they work, then you're done. I suspect you will like the
> way that they work and I also suspect changing one feedline length by as
> much as 8 feet would not affect your overall conclusion.
<snip>
> Get on the air and work guys. I bet you work more guys with two antennas
> at you disposal than just one antenna, and I bet when you do
> top-bottom-bip-bop* tests you will find some times when some stations
> are lounder under some of those four conditions. I suspect this would
> be the same conclusion whether your coaxes are the same length or
> different lengths by 4 feet or 8 feet.
>
Exactly this. You can have a few jumpers of some shortish length (1/4
wavelength ish) and try substituting them in and see what happens.
Here's the deal..
For phased arrays you can have fairly large phasing errors and the
*gain* doesn't change much. What changes is the depth and position of
the nulls. So the whole deal on the BIPBOPTOPBOT thing is that you're
moving the nulls around to suppress the stations you don't want to hear,
while slightly changing the gain in the directions of the stations you
DO want to hear. (for that matter, most likely, what you're doing is
moving the null OFF of the desired station).
So, you could model and measure and carefully calibrate your system (you
*are* allowing for the thermal expansion effects, aren't you <grin>),
and maybe not have any better effect than you would by essentially doing
the "try it and see".
For what it's worth, having two SteppIRs makes for a very interesting
scheme. You could feed with fixed lines, and just bump the driven
element a bit shorter or longer to shift the phasing. I don't recall
the exact numbers, but I seem to recall that a 1% change in length off
resonance is about 5 degrees in phase. 10% is about 30 degrees.
There will be all sorts of interactions among the elements, so you
really need to model it, but it's an interesting approach.
------------------------------
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