[TowerTalk] OptiBeam OB2-40M

[Jim Brown]

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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