[TowerTalk] Mosley antenna problems

[K7GCO@aol.com]

  In a message dated 19.01.00 13:07:18 Pacific Standard Time, 
alwilliams@olywa.net writes:
  << 
   Several posters have offered low SWR as an indication that
   a particular beam is good.  A 50 ohm resistor would be even
   better?
    k7puc >>
 
   The similarity if a Rotatable Dummy Load and the TA-33 has been mentioned 
several times in TT Posts. One even said "it won't radiate."  It's a humorous 
and a serious problem of compromise design at the same time.  I just saw 
where a TA-33 Manual even gave 2.5 dB gain on 15M.  That's not exactly a 
beam.  New hams often associate low SWR and good bandwidth as a great 
desirable feature and it is--if it's not obtained by compromise design.  With 
yagi antennas, LOW SWR with DIRECT feed (no gamma, T or 1/4 WL matching 
section) is the result of a "detuned" beam in order to achieve low SWR and 
more bandwidth. That means LESS gain and F/B.  Yagi DE impedance's range from 
20-35 ohms for typical tunings that give good patterns.  Do not evaluate a 
direct fed yagi based on SWR.  It's possible to have good bandwidth with 
higher gain beams with compensating networks.
     I had a W8CC 5 element yagi with a 28 ohm feedpoint. It has over 10 dBi 
gain and 30 dB F/B over a wide frequency.  Hoping to get greater bandwidth 
and a balun without toroids, I used a FD of such diameter ratios (3/4" & 
1/8"-3" spacing) that I had a balanced 200 ohm feedpoint.  I used the 1/2 WL 
of coax connected to the 50 ohm coax in such a way to give a 4-1 step up 
balun and no toroids.  I thought I had a SWR curve broader than I'd had with 
a Gamma.  I then used a Split DE using a 1/4 WL matching stub of 2-75 ohm 
coaxes paralleled to get a 35 ohm 1/4 WL stub.  I got 1:1 and a SWR curve 
well over twice as wide as the FD.  Talk about a surprise.   
   The 1/4 WL matching stubs used to step up the 28 to 50 ohms, creates the 
opposite reactance as does the DE thereby increasing the bandwidth.  The 
resistive part of the Antenna Z doesn't change anywhere near as fast as the 
reactance.  It's a great design with toroid donuts on it to stop the RFspill 
over down the coax.  Wrapping the coax stub in a coil does virtually nothing 
in the high current area next to the feedpoint to prevent RF spill over 
contrary to those who say it does.  In the FD configuration, the 1/2 WL of 
coax used for the 4:1 step up balun creates the same reactance above and 
below the resonant frequency as the DE and lowers the bandwidth.  It even was 
more dominate in decreasing the bandwidth than the FD DE which increases the 
bandwidth.  You get a summation of the 2.  This 1/4 WL matching stub design 
gave great bandwidth of slightly less than 1.5:1 at 28 and 29 MHz with a 
direct feed 1/4 WL stub AND little drop of Gain and F/B over 1 MHz and that 
is rarely seen in yagi's.  All-Driven Arrays can do this and more.
     Quads have higher Z's and can often be fed directly with a 1:1 Balun but 
not always.  A 3 element quad really peaked for gain can have a DE Z as low 
as 35 ohms.  2 element quads can range between 75-130 ohms depending on the 
tuning and spacing.
      Evaluating performance of a Yagi antenna by SWR when directly fed (no 
other networks) is totally misleading for overall performance.  You still 
have to evaluate the pattern compared to a standard beam tuned for reasonable 
gain and F/B with any feed system.  There are indeed some "Rotatable Dummy 
Loads" out there that the Ham Learning Curve of Ham newcomers must waste 
money on in order to find out.  The Moral of the story is "not to expect too 
much from a directly fed yagi."  Many are still looking. 
      If any ham ever uses a well designed and simple 2 element quad, he will 
have what I call the "K7GCO Standard Of Beam Performance".  Instead of a 
dipole as the gain reference for all antennas (it has no F/B), a 2 element 
quad tuned for gain or F/B is the best reference of all for beam antennas for 
initial Gain AND F/B AND initial money spent, height necessary for 
outstanding performance, rotator and tower needed to support it.  A yagi and 
physical supports for additional performance will cost you 6-10 dB MORE 
MONEY.  There is no other antenna in the Antenna Kingdom for its boom length 
(.125-.15 W/L) and simplicity that will beat it for gain and F/B.  It takes a 
yagi of 2-3 times longer boom length and another element to beat it.  All 
this data was published by Lindsey many years ago.  The lower the frequency, 
the more significant that is mechanically and financially.  
   I've done the comparisons many times on the air side by side and in Eznec. 
There are 2 element quad configurations that will give over 50 dB F/B just 
for starters.  I've seen only one 2 element yagi (Moxan) pattern anywhere 
near that high.  A quads one soldered joint per element doesn't deteriorate 
electrically either as do yagi joints.  There is no "Rheumatoid Arthritis of 
the Joints" with quads that needs Penatrox rubbed on it as do yagi's.  I hear 
that Penatrox works on stiff knees also.    
k7gco
  

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