[TowerTalk] 80m 4Square Dipole VSWR question

[Yo3ctk]

Tom,

I read from your comments that the Comtek box doesn't work and cannot work. 
Is my reading correct?

sheesh, I just bought one, based on favorable comments read on the net....

73 de Mike, YO3CTK

----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "Paul Playford" <w8aef at worldnet.att.net>; "'D. Rodman, MD'" 
<rodman at buffalo.edu>; "'towertalk'" <towertalk at contesting.com>
Cc: <topband at contesting.com>
Sent: Thursday, November 18, 2004 5:03 AM
Subject: Re: [TowerTalk] 80m 4Square Dipole VSWR question


>> I would not expect a current balun to change the resonant
> frequency (0 ohms
>> reactance) of a dipole.
>
> Why not Paul?
>
> Anything that changes the common mode impedance of the
> system can change the resonant frequency of an antenna
> system. As a matter of fact if the feedline is not matched
> changing the length of the feedline will change the apparent
> resonant frequency of the antenna.
>
> This is why I didn't feel warm and fuzzy about the
> "resonance" mode in the MFJ analyzer. Just like mismatch
> loss, it is useless except under very specific conditions.
>
> I found the velocity factor on the 75 ohm coax I used on
>> my 40m 4 square to be closer to .7 than the .8 the
> manufacturer specified
>> and those lines wound up being almost taught between the
> box and feed
>> point(s).  The v/f on the lines for my 80m 4 square were
> very close to mfg
>> specs.  It has been my experience that once I measured the
> V/F of a roll of
>> cable it remained constant throughout the roll, but
> different rolls may have
>> different V/F's.
>
> If they do have different Vf they also have different
> impedance. Unless  conductor size or spacing is changed to
> compensate of course.
>
>> I'm treading on thin ice here so please don't take the
> following as gospel.
>> Something else that may be distorting your observations
> would be the 1/4
>> wave (75 ohm) impedance transformer/phasing lines.  With a
> feed point Z of
>> 35 ohms (ground plane) the Z at the other end of the line
> will be 165 ohms.
>> Assuming that the phase shifting box was designed to drive
> a ground plane
>> array this would require the 0 degree port match 165 ohms,
> the 90d port
>> match 82 ohms (two elements in parallel) and the 180d port
> match 165 ohms.
>
> That ignores mutual impedances. The actual design Z0 should
> be much different.
>
>> With a dipole array there won't be much, if any, impedance
> transformation
>> (feed point Z of each dipole may be 75 ohms +/-), the same
> shifting box
>> should be designed to match 75 ohms, 36 ohms and 75 ohms.
>
> Not quite. Aside from earth effects, mutuals can make the
> dipole impedance quite different. The front element can
> easily be above 75 ohms and transform down, while the null
> direction element will be below 75 ohms and transform
> upwards.
>
> Lets just assume the mutuals cause the front element to
> double, the rear element to halve. With a Marconi element
> the 35 becomes 70. It hardly transforms through the 75 ohm
> cable. The rear becomes 17.5, and transforms to 321 ohms.
>
> Now let's just say a dipole would be 75 ohms and have the
> same mutuals. The front would become 150 ohms and transform
> down to 37.5. The rear would become 37.5 and transform up to
> 150 ohms.
>
> So we have the front changing from ~80 in a Marconi element
> array to 37.5 with dipoles.
> We have the rear changing from ~321 ohms in the Marconi
> array to 150 in the dipole array.
>
> I'm afraid the idea of considering the non-coupled impedance
> of the elements as the impedance the phasing unit sees as a
> design value is just like peeing while on a tall tower. You
> never know exactly where it will land.
>
> You really have to know the design impedance of the phasing
> system, and then model the system to match. The ports can't
> possibly work as planned if they are designed while ignoring
> mutual impedances.
>
> For an example, consider my 160 meter 4 square. It has 1/4
> wl Marconi elements that have about 34 ohms J0 impedance.
> There are 1/4 wl lines to the phasing system.
>
> When driven, the impedances at the elements become:
>
> Front element : 14.42 j59
> Middle elements 25.5 j 0.8
> Rear element 1.7 j 0.8
>
> NONE are 34 ohms!
>
> The impedance at the phasing unit is:
> Front 11.87 -j40
> Mid 94.5 j0
> Rear 774 -j205
>
> My array is not a conventional 0 90 180 shift array, so you
> can't use these values. But rest assured if I had dipole
> elements and simply changed the line so the "non-mutual
> coupled" elements had the same Z0 as the 34 ohm elements
> through 1/4 wl of  50 ohm cable =  72 ohms (the cable has
> .2dB matched loss, so the transformation is not to 73.5
> ohms) and I used 72 ohm elements through 75 ohm cable to
> have the same non-mutual Zo, my phasing would be as accurate
> as a rubber watch.
>
> The thing that allows us to do poor designs and remain happy
> is the fact these antennas work to some extent regardless of
> we mess things up. Nobody is likely to notice a gain loss of
> 2-3 dB and a F/R loss of 20 dB. Most people are happy with
> 20 dB F/R and a few dB less gain.
>
> The only thing really important is how hot the dump resistor
> gets, what the SWR meter says, and if the array has mediocre
> F/B.
>
> 73, Tom
>
>
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