[TowerTalk] OptiBeam OB2-40M

[VE6WZ_Steve]

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> 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