Topband: 4 square receiving array

[markwa1ion at aol.com]

<<
From: "Tom Rauch" <w8ji at contesting.com>
...

>Mark, WA1ION, has done some
> experiments using just transformers to "match" various
> antenna types.
> I also read that an antenna could be "matched" by using a
> transformer
> that steps up the impedance to the same value as the
> antenna reactance.
> In other words, if a short vertical had a reactance
> of -1200 Ohms, it
> could be "matched" to 75 Ohms with a 1-16 ratio
> transformer.

No, it cannot. 75 ohms reactance terminating a 75 ohm line
is an infinite SWR.

A reactance can only be converted to a resistance by
applying an opposite sign reactance. say you had a short
whip with 35 ohms of ground terminal resistance. If you
applied  35-j1200 to a perfect 16:1 transformer the result
would be 2.19 -j75.
>>

I fully agree that for a 4-square system with its exacting need for 
stable gain and phase per element, correctly-matched (e.g. tuned and 
somewhat Q-spoiled) passive elements, or smaller active elements (with 
amplifiers that also present the correct output impedance to the line) 
are mandatory, especially in a permanent set-up.   Active antennas pose 
more of a challenge because the need to get DC power to them: this 
reduces the ease of using standard feedline isolation transformers to 
prevent the feedline's interaction with the antenna.  The DX 
Engineering active system meets these challenges.

With four antennas there are simply too many degrees of amplitude / 
phase freedom to try to keep things controlled with operating-position 
phase-shifting and amplitude-balancing circuits having wide adjustment 
ranges.

Obviously a 4-square set up for transmitting as well as receiving would 
be constructed quite a bit differently in terms of height of the 
verticals, quality of the ground system, and robustness of any 
transformers, power splitters, coils, and capacitors used.

The transformer "matched" antennas do have some merit, however: 
primarily in two-element phased arrays, or operated singly.

Dallas Lankford has probably done the most research into broadband 
transformer matching of receiving verticals.  A recent article "15' 
Relay Tuned Noise Reducing Vertical Antenna" at 
"http://www.kongsfjord.no/dl/Antennas/15%20Foot%20Relay%20Tuned%20Noise%2
0Reducing%20Vertical%20Antenna.pdf" is worth a read.  It uses a relay 
to set two wide-bandwidth transformer settings: 0.15 - 8 MHz and 8 - 30 
MHz.  For operation below 8 MHz, the relay can be eliminated.  It is 
understood that a precise conjugate match to the feedline is not being 
achieved, but sufficient signal-to-noise sensitivity can be had.  
Because the antenna impedance is already high, the ground requirements 
at the antenna are modest: a 4 to 8 foot ground rod, possibly 
augmentable with a few radials if the soil is poor.  Balanced feedline 
is used to go from the antenna site transformer back to a second 
transformer positioned at the receiver or phasing unit input.  The 
article doesn't mention the maximum length of twinlead feedline but one 
of Dallas' other articles states that at least 100 ft. (30 m) can be 
used successfully with minimal loss below 2 MHz.  Balanced-feed and 
transformers on each end tend to minimize "feedline-as-antenna" type 
interactions.

Single-turn loops were also "matched" after a fashion in another 
Lankford article "Big Untuned Single Turn Air Core Loop Antennas" at 
"http://www.kongsfjord.no/dl/Antennas/Big%20Air%20Core%20Loop%20Antenna%2
0II.pdf".  Again it is more of an exercise in maximizing signal 
transfer using broadband transformer techniques and cannot be construed 
as true conjugate (i.e. narrowband) matching.

For a two-element phased array (rather than a 4-square), the long-term 
stability of the impedance isn't quite as critical, especially when the 
two antennas are feeding a phasing unit having adjustable phase and 
amplitude.  This is particularly true on temporary installations, such 
as during coastal DXpeditions.

Mark Connelly, WA1ION - Billerica, MA
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