Depending on the design of your phasing network, the per-element VSWR will
not go down when the elements are phased. What happens with hybrid couplers
like Comtek's is that the reflected power is dumped into a dummy load, so
you see nearly flat VSWR at the transmitter.
However, when you phase the array, the resonant frequency will *increase*.
You must tune each element, while all others are disconnected, lower than
the desired resonant frequency. The rule of thumb I've seen is 5%-10% below
the desired resonant frequency. So, if you want the array's resonant
frequency to be 3.800, you would tune each element somewhere between 3.400
and 3.600.
BTW, the VSWR figures below are so close that I wouldn't bother trying to
make them exactly the same. However, 3.800 is too high for the resonant
frequency -- it would put the array above the 75m band.
73, Dick WC1M
> -----Original Message-----
> From: PY2YP [mailto:py2yp@pobox.com]
> Sent: Monday, June 16, 2003 7:47 AM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Adjusting a 4 square
>
>
> Hello,
>
> I do have a problem that, due to a lack of knowledge, I can't
> find the solution.
>
> I did measure the 4 towers of my 4 square when they were not
> phased, e.g. them all were disconnected, and fed each one at
> a time to found out the following VSWR values:
>
> Tower 1: 3.800 1:1.41
> Tower 2: 3.800 1:1.59
> Tower 3: 3.800 1:1.54
> Tower 4: 3.800 1:1.48
>
> I have a spreadsheet (57 kbytes) with the complete measured
> values and a graph as well.
>
> All towers have an aluminium tube at the end which allows to
> either make it shorter or longer. The question is: based on
> these values how long do I have to increase each one, e.g.,
> towers numbers 2, 3 and 4, in order to have them all set to
> the same VSWR value, I?d say 1:1.41.
>
> As far as I know the 4 towers when phased will have low VSWR
> values due to the mutual impedance. Am I right?
>
> Thanks in advance for any help.
>
> 73 DX de PY2YP - Cesar
> http://www.qsl.net/py2yp
> py2yp@pobox.com
>
>
>
|