Using 180 degree phasing has several advantages including the advantage
of being easily implemented, especially since you can get a broadband
version by using a transformer. However if you are talking about a four
square, using 180 phasing will produce less gain (important for
transmitting purposes) and higher take-off angles (important for both
receiving and transmitting than using optimized phasing.
In the case of receiving antennas this line termination/impedance
problem is not as big of a problem as it is for transmitting antennas.
The feedpoint impedance of most of these receive antennas is determined
mostly be the termination resistor and the antenna shape and it doesn't
change a lot with frequency.
The terminating resistors for EWEs and K9AYs isn't a function of line
phasing. It's a function of antenna shape. These receiving antennas are
not very critical of termination value. The termination resistor will
change the depth of the null, and it can be tweeked to produce some
exceptionally deep nulls. However this null, three-dimensionally, is
very skinny so tweeking the null from say 20 db to 40 dB doesn't really
change the signal-to-noise ratio of the antenna significantly. If a
signal happens to drop into the hole by coming in at the right elevation
and azimuth angle it may give you the feeling that the antenna is
working better if the null is very deep. All that is really required for
good signal-to-noise ratio is to get fairly close. This is true for the
K9AY, EWE, flag, or Pennant. They all work the same way.
Jerry, K4SAV
Pete Smith wrote:
>Interesting discussion, Jerry - I wish Tom would chime in.
>
>Another consideration I have been wondering about - there is a graph in
>ON4UN's book (3d edition, page 11-7) that shows the degree of phase shift
>versus line length in degrees, when the phasing line is not terminated in its
>characteristic impedance. I suspect that many of us, particularly when
>seeking a compromise terminating resistor on a EWE, K9AY etc. aren't THAT
>careful about matching. This is one of the reasons I was attracted to
>180-degree phasing - it appears to be unaffected by mismatches, according to
>ON4UN.
>
>73, Pete N4ZR
>
>At 04:42 PM 12/20/2006, K4SAV wrote:
>
>
>>A couple of words about RDF (Receiving Directivity Factor) and DMF
>>(Directivity Merit Figure). Both of these may be used as a kind of
>>figure of merit for receiving antenna performance, but neither of these
>>fully characterize everything that is important in determining
>>signal-to-noise ratio for a receiving antenna. A degree of judgment
>>needs to be applied in addition to looking at the number values.
>>
>>RDF was originated by W8JI and it is calculated by measuring the forward
>>gain in the "desired" direction minus (in dBi)the total average gain of
>>the antenna. This is easy to determine because EZNEC will calculate
>>both of these. It's generally a good parameter, but there are a few
>>problems.
>>
>>The first problem is, what is the "desired" direction. Most experienced
>>DX'ers will have a good idea as to what is needed, but the inexperienced
>>ham won't, and the desired direction may be different for different
>>applications. But maybe that's OK on an individual basis because
>>conditions may be different for different people or for different
>>applications. For example one person may choose 25 degrees elevation,
>>and another person may choose 15 degrees elevation as the desired angle
>>and the direction to be in-line with the lobe. I saw a write-up on one
>>guy's website where he used the desired direction as whatever the
>>elevation angle was for peak gain. That made a low dipole look really
>>good. The point is that numbers generated by different people are going
>>to be different for the same antenna.
>>
>>The second problem is that the parameter makes no distinction for lobe
>>shape. In other words, you can squish the forward lobe into any shape
>>you desire as long as you don't change the "gain in the desired
>>direction" and overall average gain (just like a balloon) and the RDF
>>number won't change. The shape however will change the S/N ratio.
>>
>>The third problem is that a lot of the response in the forward lobe can
>>be moved to the rear (just like the balloon analogy again). If the
>>"gain in the desired direction" and average gain are not changed, the
>>RDF number won't change. Front-to-back ratio MAY be more important than
>>a wider front lobe to some people.
>>
>>The point is that the signal-to-noise ratio of the antenna may change
>>significantly and the RDF number remain the same.
>>
>>ON4UN recognized some of these short-comings in the RDF parameter, and
>>developed his own parameter, DMF. DMF is the gain in the desired
>>direction minus (in dBi) the average gain in the rear half hemisphere.
>>This is a little different from RDF, but it has some of the same
>>problems. We still have this "desired" direction, so it has the same
>>problems as listed above which may make the number different for
>>different applications. The DMF does give a lot of weight to
>>front-to-back ratio, and so precludes the case of "squishing" some of
>>the forward lobe into the reverse direction without the number changing.
>>It doesn't help the problem of squishing the front lobe into various
>>shapes however. In other words, the front lobe could be horizontally
>>very wide and vertically very narrow, or you could make it very narrow
>>horizontally and very wide vertically. Both would have the same DMF
>>(and RDF). Clearly the lobe with narrow vertical response would be much
>>more desirable.
>>
>>When I first looked at these two parameters and saw the problems, I
>>started developing my own method of measurement, however I never
>>finished it. With a little expertise in recognizing what is important in
>>terms of the shapes of the response curves, you should be able to
>>determine which is best. This however doesn't give you a number, and so
>>comparison between a large number of antennas becomes difficult. I wish
>>I had finished my method of measurement now, because it would be nice to
>>have something that provided better characterization.
>>
>>Another item of importance but not necessarily a large player in
>>determining the S/N is the location of the nulls. These may be important
>>for rejection of QRM from certain directions. That may give more weight
>>to obtaining a good front-to-back or a null at 90 degrees, depending on
>>where you are located and what QRM you would like to reject.
>>
>>Jerry, K4SAV
>>
>>Terry Conboy wrote:
>>
>>
>>
>>>I've followed with interest the discussion about phased arrays of
>>>K9AYs vs. 4 squares, such as the DX Engineering receiving array. I
>>>decided to model a few variations, and they back up K4SAV's & NI1N's
>>>comments pretty well. As the basis for performance comparison, I used
>>>the RDF (receiving directivity factor) employed by W8JI. Here are the
>>>results:
>>>
>>>Antenna RDF, dB
>>>========================
>>>Whip 4.7
>>>K9AY 7.3
>>>EWE 7.4
>>>
>>>4Sq Whips 90* 10.1
>>>4Sq K9AY 90* 10.2
>>>4Sq EWE 90* 10.2
>>>
>>>4Sq Whips 120* 11.6
>>>4Sq K9AY 120* 11.4
>>>4Sq EWE 120* 11.4
>>>
>>>1 wl Echelon-Bev 11.5
>>>1.5 wl Beverage 11.6
>>>
>>>[The 4 squares all use 1/4 wl spacing. The 120 degree phasing system
>>>is very close to what results from using the crossfire phasing scheme
>>>recommended by W8JI and DX Engineering at that spacing. The crossfire
>>>system is broadband and can be used over several bands, and the
>>>current phases will vary accordingly.]
>>>
>>>The bottom line is that a single K9AY or EWE (which are essentially
>>>equivalent if properly installed) is a better receiving antenna than a
>>>single whip vertical, but using them in arrays may be much more
>>>complex than required for good low noise receiving performance. The
>>>only notable improvement for the arrays of EWEs or K9AYs is a
>>>reduction in the off-axis back lobes, which could be useful for
>>>attenuation of some low angle QRM. On the other hand, as K4SAV
>>>describes, the array of verticals has a null at zenith, which could
>>>help attenuate lightning static and NVIS QRM.
>>>
>>>It's also interesting that all of the 4 Square arrays are very similar
>>>in performance to a 1.5 wavelength Beverage or an echelon of 1
>>>wavelength Beverages (staggered by 0.186 wl) with 135 degree phasing
>>>(although the back- and side-lobe structure is different.)
>>>
>>>Incidentally, models of the DX Engineering array actually show
>>>slightly better RDF (a few tenths of a dB) at closer spacings than 1/4
>>>wl (with appropriate phasing). I suspect the arrays of EWEs and K9AYs
>>>would be similar.
>>>
>>>73, Terry N6RY
>>>
>>>
>>>
>>>
>>>
>>>
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