Hi Jeff,
No question that SWR changes with several elements of design, and a good
match in the shack is needed to make some power amps happy. I'm no Yagi
design guru, but I've done a few for special purposes that worked, and
played with the design variables that you and Rick have noted. One was a
2-el Yagi for 20M on a 16 ft boom, that W6GJB built with elements that
folded at their point of attachment to the boom, so that it could go
down the road on the pneumatic mast on his CQP/7QP/FD contesting
trailer. k9yc.com/7QP.pdf shows the trailer, before that antenna was
added to his bag of tricks. That construction demanded direct feed, and
I managed to achieve that with low SWR and decent gain-bandwidth by
playing with element spacing and lengths.
73, Jim K9YC
On 5/23/2021 1:20 PM, Jeff Blaine wrote:
Ah, but it can be, just not in the way guys generally think about...
A 2 element beam gain increases with coupling. And that in turn drives
the feedpoint Z downward.
In fact this characteristic can be used for tuning a 2 element beam. Set
the elements to the same length, then start shortning the DE relative to
the reflector (or the director relative to the DE if you prefer that
design) and sweep the general frequency range with the VNA, keeping
track of the R at X=0. When the R gets to about 20 ohms, that's not a
bad sweet spot. Then center the yagi on frequency by trimming the two
elements to the same amount each time, hopefully getting the X=0 in the
middle of where you want it. Top off with a hairpin to bring a 50R
direct feed and voila, that beam is going to be a very good performer,
at least as a 2-element type goes. All that assumes you have the
antenna well off ground relative to the bandwidth of course.
73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com
On 5/23/21 2:11 PM, Jim Brown wrote:
YES! SWR is NOT an indicator of antenna performance.
73, Jim K9YC
On 5/23/2021 9:40 AM, Richard (Rick) Karlquist wrote:
On 5/23/2021 7:29 AM, VE6WZ_Steve wrote:
I have received a few direct email questions about this thread…..
Just to be clear, the reason for inductor switching at the Yagi
elements is NOT only to get a “good, low SWR" match at the driver.
The “SWR” and “matching” at the feedpoint is really NOT the issue.
The real issue is tuning the REFLECTOR across the band. That is
what determines the gain and F/B performance.
The PARASITIC element needs to be incremented across the band to
maintain maximum gain and F/B performance.
Yagi SWR has very little to do with performance.
Steve, ve6wz
Another great posting from Steve. It seems like this point
has to be constantly repeated on TT to counteract marketing
hype and wishful thinking. When I first got my MonstIR,
I did some experiments where I set the tuning for 40 CW
and then QSY'ed to 40 phone, without retuning the MonstIR.
Of course the SWR went way up. I then used the custom
tuning feature to tweak just the driven element for SWR.
This did improve the VSWR somewhat, but the gain and F/B
were still lousy. SWR bandwidth doesn't equal pattern
bandwidth. And the MonstIR is a full size 3 element
design (yes, 70 foot elements), so it will tend to do
better than a shortened 2 element.
Rick N6RK
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