About 50 years ago I started building receive antennas, looking for and hoping
to find the "magic solution". About 10 years ago, I finally decided there was
no magic, no free lunch.
A serious effort was started to objectively evaluate various RX antenna designs
using controlled experiments and many data points often taken over years of
time. This involved on-the-air real-time comparisons between the antenna being
evaluated and one or more known performance antennas.
After moving from a suburban lot in AZ to this farm in WA, the need for
"compact" RX antennas was reduced but not eliminated because the 1100 foot
beverages tend to not work well when balled up in the tracks of a D4 Cat. After
a few seasons when the farming activities caused the Beverages to be erected
late or not at all, I started looking at options that I could erect inside the
"compound" and keep up year round.
The usual Flags, K9AY,s, EWE's, and various forms of short Beverages were tried
and used with some degree of success but each had logistics problems. Simply
stated, the structures were always in the way. So I started looking at small
vertical arrays. The following is what I have observed so far.
First of all I needed a "gold standard" to compare to for performance
evaluations. The Flag has proved over the years to be the absolute best antenna
for this application due to the fact that it is almost immune to degradation
caused by nearby structures. In AZ I had one Flag installed UNDER the elevated
radials for the 160 and 80 towers and it worked as well as the one out in the
clear. Try that with your "Binford 8000 Super Array" sometime!
Additional "standard" antennas used during the small vertical array evaluations
were two 400' "permanent" Beverages, and various EWE's. In addition, I have a
signal source mounted on my pick-up so that I can move it around and determine
azimuth pattern skewing caused by interaction with the other structures on the
property.
I started with a two element end fire array modified a bit from W8JI's design
to allow for physical limitations caused by the location where it needed to be
erected. Maybe this difference caused some of the problems I had. I spent
several months trying to get this two element "tuned" array to work to my
satisfaction but never succeeded. Some of the problems have already been
mentioned. I had a terrible time with the tune networks. I could easily tune
them to frequency and adjust for identical feedpoint impedances but that was no
guarantee that the phase/amplitude output from each element was what it needed
to be. The ground radials were very much a problem and re-tuning was required
every time I tripped over one of the radials. I finally gave up on the "tuned"
system. I could most likely have made it work but it just wasn't worth the
effort.
Jumping ahead a bit, I recently tried using 16:1 transformers for a two element
array with good results. K7TJR reports phase/amplitude errors using these
transformers but for a simple 2 element array, the errors are not serious
enough to degrade the S/N performance. The F/B is less than what I see using
HI-Z amps at the feed point but the overall performance is essentially the
same. Simple 16:1 Transformers on the usual binocular core is a good cheap way
to go for a simple array, but don't even think about it for something more
complex. The little 2 element array (50 foot spacing) using these transformers
provides decent performance from the broadcast band up through 40 meters when
used with a "crossfire" type phasing unit.
When I discovered K7TJR's Hi-Z amp kits, I started to play with the small
vertical arrays again. Using Lee's system completely eliminated all the twitchy
problems I had with the "tuned" system and I soon put a 4-sq together. I used
only 50 foot spacing and a homebrew controller based on K7TJR's design. I did
this just before summer Solstice.
During the summer months, the only common window I have with DX is at 2-3 AM,
thus limiting the number of data points I can gather up. What I saw was the
4-sq did not work on DX at all, with the Beverages, and even the TX vertical,
doing a better job of hearing DX. This in spite of countless hours of checking
patterns using AM BC, WWV, and my pick-up truck signal source. Everything
worked as designed on "local" daytime signals but not on DX. This continued for
several weeks and then like magic the array started working. The further away
from Slostice, the better it worked. I concluded once again that DX signals
arrive at a very high angle far more often then "conventional wisdom" would
have one believe.
With the 4-sq installed near (50 feet) the TX vertical, there is definitely
pattern skewing even though the vertical is detuned during RX periods. If you
think detuning makes the TX antenna invisible, think again. If you are very
lucky, detuning will reduce the interaction enough to push that one problem
down below the level that can be detected in the "noise" caused by all the
other structures within 10 wavelengths!
The more complex the array, the more pattern skewing will occur due to other
structure. Computer modeling will NOT adequately predict how any given RX
antenna will perform because every installation is different and every RX
antenna reacts differently to nearby structure. The simple arrays such as a
Flag are usually not degraded (percent-wise) as much as the more complex
arrays. It is entirely possible, for instance, for a RX 4-sq erected in a
compromise location to be consistently beaten by a Flag! OTOH, you won't know
until you try it! In my case, even though I can see significant pattern
skewing, the "effective RDF" is not dramatically reduced and the resulting
system, although not optimum, is still pretty good.
To summarize:
"Tuned" elements are not something I would recommend due to being "single band"
and difficult, relatively speaking, to get working.
Transformer matching is quite usable for simple arrays, but not practical for
anything as complex as a 4-sq or better.
Hi-Z amplifiers can be a problem. Some are prone to IMD, some blow up when you
transmit. I have not used the DXE amps so can't say about them, but I know that
Lee, K7TJR, has solved all these issues and more in his design. His are good
enough to eliminate all the usual objections to using active element matching.
Phasing box / controller designs which do not use some form of "crossfire"
phasing are to be avoided. If not "crossfire" or functionally equivalent, they
are only single band and they will there for be more critical of phasing
"errors". Wide band operation is essential if only because you need to be able
to check Radio Australia on 2485 kHz from time to time!
There is no formula to determine what RX antenna to invest in. The best advice
is to invest in them all! Not only do you have to consider the individual
design requirements, you also have to consider propagation anomolies for your
specific location (which can be dramatic), but also ALL the surrounding objects
and noise sources for your location, and workmanship as well. I have seen
perfectly good RX antenna performance destroyed just because the guy installing
it didn't have any idea what he was doing.
There is no magic, there is no free lunch. The difference between the best and
the worst when installed on site is often only a couple dB. Just because you
have a 1500 Visa bill, doesn't mean that you have the best solution. Sometimes
cheap is best!
The 4-sq results are not complete as I have only been collecting data for
several months and am still working out issues with my specific installation.
(the 500 foot cable run cause more issues than expected and the 1.5 megawatt
Loran-C station down the road is problematical) If anything significant pops up
during the next season, I'll try to report on it. Hopefully I can get the long
Beverages up soon and have another comparison point.
73, Larry
Larry - W7IUV
DN07dg - central WA
http://w7iuv.com
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