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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