[TowerTalk] OptiBeam OB2-40M

[Jeff Blaine]

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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