<<
From: "Tom Rauch" <w8ji@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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