[TowerTalk] OptiBeam OB2-40M

Sat May 22 19:19:13 EDT 2021

Steve

Thanks for the proper proper analysis...8^)

Ignoring possible SWR losses as many will unlikely go to your due 
diligence on matching which I applaud..I would argue that .5 to 1.3db is 
hardly worth the physical time and effort

I would also argue however on your side about the relays... If one 
chooses high quality relays, with plenty of margin on current and 
voltage protection and then properly mounts them in a well thought out 
(relays at the top screened drain holes at the bottom ) weather 
resistant enclosures, they will likely out live most operators ...8^)

I use an approach similar to yours to move the resonance around on my 
160 and 80M antennas. I have had only had one problem here in the� 
jungles of west central Florida which was my own fault ( Part of the 
curse of having ADD)� and I left the door to the enclosure open� over 
night� and all were exposed to the rain overnight . Things got a little 
spectacular when I dumped 1.5KW into the 160 box the next morning ...8^(

Dave
NR1DX

On 5/22/2021 5:36 PM, VE6WZ_Steve wrote:
> Dave et al,
>
> This has drifted off topic, but I dug out my 4NEC2 Cushcraft XM-240 
> models and coil calculations.
> (please be aware I have not checked the details of this modelling 
> since I did it some time ago, so there could be errors)
>
> Using Brian's coil 3.33, which is not the latest version, the original 
> CC coil with 68 turns of 12 ga wire on a fibreglass form gives a Q of 
> 258 on 7 MHz and a resistance of 1.5 Ohms.
> A high Q coil made from 3/16� Aluminium tubing, 11.5 turns, 5� 
> diameter and 5� long gives a Q of 890, and a resistance of .6 Ohms.
> (these two input values do not yield identical inductance, but are 
> close enough for estimation)
>
> In 4Nec2 I model the Yagi at 21m high.
>
> For the original CC coils I insert 1.5 Ohms in series at each of the 4 
> coils to account for the loss.
> This yields the following gain:
> 7.010-10.8 dBi �this is peak gain (SWR 2:1)
> 7.100-10.2 � � this is the 1:1 SWR point at resonance (R=48.6 Ohms)
> 7.200-9.48
>
> Using the HI-Q coil I insert .6 Ohms in series at all 4 coils to 
> account for the loss.
> This yields the following gain:
> 7.010-11.3 dBi this is the peak gain (SWR 2:1)
> 7.100-10.52 � �This is the 1:1 SWR point.
> 7.200-9.63
>
> So I calculate the gain advantage of the HI-Q coils to be about .5 dB.
> The �penalty� for getting that big bandwidth will range from .6 dB on 
> 7.100, and 1.3 dB on 7.2 MHz.
>
> In both cases (HI-Q and CC coil) the SWR 2:1 band width is from 7.000 
> to 7.280�a full 280 kHz !!!
> This huge BW is because the driver is tuned for 1:1 SWR a full 100 kHz 
> above the max gain point.
> As I said earlier, CC did this because the R is 50 Ohms�a perfect 
> match to coax and the Yagi can be �used� from the CW to SSB part of 
> the band.
>
> However, if we tune the Yagi driver closer to the peak gain point, say 
> at 7.05 (which is also the peak F/B qrg), then the feedpoint impedance 
> is only 33 Ohms (we need a matching network).
> The 2:1 SWR BW is now only 170 kHz�..from 7.000 to 7.170, so in-band 
> switching will be needed to go from CW to SSB.
>
> On both my 40m and 80m Yagis, I have a series of relay switched 
> inductors that will move the operating window across the band, so from 
> CW to SSB, I am almost always within a fraction of peak gain.
> I have 4 segments on my 40m Yagi, and 18 band segments on my 80m Yagi. 
> �All relays switch automatically using the Station Master band decoder 
> that follows the radio.
> Many have commented that relays can be un-reliable, and a maintenance 
> problem�well...my 80-40m Yagi has been in service for 18 yrs.
>
> Here is a paper I wrote explaining the 80m Yagi, that shows the 
> in-band switching system.
> https://drive.google.com/file/d/1IksvND0LfeSBcH6j7UEWkyjkIvexDIgX/view 
> <https://drive.google.com/file/d/1IksvND0LfeSBcH6j7UEWkyjkIvexDIgX/view>
>
> 73, de steve ve6wz
>
>> On May 22, 2021, at 9:24 AM, Artek Manuals <Manuals at artekmanuals.com 
>> <mailto:Manuals at artekmanuals.com>> wrote:
>>
>> Steve et all
>>
>> So what is the "penalty" in DB (gain) for the using the increased 
>> bandwidth settings on this beam vs an optimized for gain approach? 
>> For the sake of discussion ignore the F/B degradation ...just raw dBd 
>> or dBi gain comparison
>>
>> Dave
>> NR1DX
>

-- 
Dave Manuals at ArtekManuals.com www.ArtekManuals.com

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