Trent,
Elegant simplicity is the reason insulated base verticals are the "defacto
standard" for 4-square arrays. Each element is identical and most importantly
there are no reactive elements between the transmission lines and the
verticals. This makes it practical to design a relatively simple matching and
switching network for the entire array.
There are at least three big problems with adding a matching network at the
base of each vertical:
1. only two of the verticals (the two that are fed in phase) have identical
impedances. The leading and lagging phased verticals have significantly
different impedances.
2. vertical base matching networks (e.g., a gamma match) change the RF current
and phase delay to each vertical. Their affect will not be identical as the
array is switched because the impedance of each vertical changes when we switch
direction.
3. wbecause we switch our 4-square verticals in four different directions, it
is very different from AM broadcast arrays that are usually aligned for a
single pattern. Because of (1) and (2), a 4-square array with matching at the
base of each vertical (e.g., a gamma match) would require switching of three
different networks at each vertical base.
In summary: find a way to insulate the bases of your verticals. It will save
you a huge amount of time and money (and maybe a masters degree in electrical
engineering and several years of field experience with adjustment AM
broadcasting arrays) if you try to use a gamma match on each vertical!
73!
Frank
W3LPL
---- Original message ----
>Date: Sun, 17 Dec 2006 14:54:12 +1000
>From: "Trent and Lorraine" <vk4ti@sampson.net.au>
>Subject: Re: [TowerTalk] 80m 4-square materials
>To: <TOWERTALK@contesting.com>
>
>
>Greetings all, Could anyone enlighten me ? : Is there any practical reason
>that free standing towers utilised in a gamma match configuration are not
>used for 4 squares ? Locally there are heaps of old free standing tv towers
>that are around the 50 ft mark
>
>Trent VK4TI
>
>Trent Sampson
>[PO Box 1647 Toowoomba Qld 4350] VK4TI
>
>
>-----Original Message-----
>From: towertalk-bounces@contesting.com
>[mailto:towertalk-bounces@contesting.com] On Behalf Of john@kk9a.com
>Sent: Sunday, 17 December 2006 12:15 PM
>To: TOWERTALK@contesting.com
>Cc: David Robbins K1TTT; mike@mallardcove.com
>Subject: Re: [TowerTalk] 80m 4-square materials
>
>I don't know that Array Solutions still carries the 80m vertical. ZeroFive
>Antennas http://www.zerofive-antennas.com sells one that looks very
>similar to the Array Solutions freestanding vertical for $2200. I built a
>54' freestanding vertical at P40A for considerably less. For K1TTT's
>installation, I would consider using 3" diameter aluminum tubing guyed at
>two levels. The larger top sections will improve the bandwidth and I think
>it's a strong antenna than can be built for a reasonable price. I believe
>that K4JA built his 80m 9 circle vertical array this way.
>
>John KK9A
>
>
>To: <k1ttt
>Subject: Re: [TowerTalk] 80m 4-square materials
>From: "Mike Fatchett"
>Date: Sat, 16 Dec 2006 09:16:16 -0700
>List-post: <mailto:towertalk@contesting.com>
>
>I believe that Array Solutions also has full sized 80m verticals.
>
>
>David Robbins K1TTT pravi:
>
>>I am trying to decide what to use to build 4 new 80m verticals for a
>>4-square upgrade here. My criteria are:
>>1. full size 1/4 wave verticals, no top or base loading.
>>2. must be able to standup to new England winters at 2000' with ice and
>>wind 3. must be new material, I don't want to go scrounging for used
>>tower or pipe.
>>4. prefer only 1 guy level, but will do 2 if necessary.
>>5. prefer to stand up complete, but will climb to add top section(s) if
>>necessary, crane or truck access will be very hard to this spot.
>>6. prefer something that can be delivered to here, truck delivery is
>>not a problem, I don't want to have to haul long stuff myself.
>>
>>So, what might fit those criteria??
>>
>>
>>David Robbins K1TTT
>>e-mail: mailto:k1ttt@arrl.net
>>web: http://www.k1ttt.net
>>AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
>>
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