Excellent point Steve and very well stated. The Cushcraft is a good
upgrade to an inverted V but it definitely has compromises. I use OptiBeam
40m beams which have high Q coils and a hairpin coil. Your relay design is
really the only way to make a shorty 40 efficient and broad banded (I do
not consider the Moxon to be a true shorty 40).
John KK9A
Sent via the Samsung Galaxy 7 edge, an AT&T 4G LTE smartphone.
On Wed, Mar 28, 2018 at 12:27 AM, VE6WZ Steve <ve6wz@shaw.ca> wrote:
> The large BW and 50 ohm feed point of the XM240 is partly due to the
> loading coils, but mostly because of the reflector element tuning.
> With any Yagi, as the max FB QRG gets closer to the minimum SWR qrg
> (resonance), the BW will crash. Cushcraft designed this Yagi to provide an
> easy match and a big BW, but sacrificed both gain and FB. Specifically, if
> the reflector is tuned for max FB at or above the design min SWR frequency
> the bandwidth will be very narrow. This is because the elements are very
> tightly coupled. Also, the feed point impedance will be very low.
> The XM-240 has the reflector tuned much below the design centre SWR min
> frequency. In other words, if you shortened the reflector element of the
> XM-240, (but left the driver unchanged) the SWR would be very high because
> the impedance would be much below 50 ohms and the bandwidth would be very
> narrow, but the FB and gain would be improved.
>
> I rebuilt my XM-240 with hi q coils, but I also shortened the reflector to
> maximize gain and FB based on NEC modelling. I require a helical hairpin to
> match to 50ohms, since the feed point impedance is around 25 ohms. The
> bandwidth is so narrow I have built band-switch boxes at each element, each
> with 4 relays to add inductance to cover all of 40 m. This is the same
> system used on my coil loaded short 2el 80m Yagi.
> The XM240 is s proven performer even with the lossy coils, and like any
> commercial product, simplicity and universal appeal (broad bandwidth) will
> always lead to a compromise.
>
> De Steve Ve6wz.
>
>
> From Babcocks iPhone
>
> > On Mar 27, 2018, at 7:25 PM, <john@kk9a.com> <john@kk9a.com> wrote:
> >
> > I should have said lossy loading coils may contribute to this exceptional
> > bandwidth.
> >
> > John KK9A
> >
> > -----Original Message-----
> > From: john@kk9a.com [mailto:john@kk9a.com]
> > Sent: Wednesday, March 28, 2018 01:24
> > To: 'towertalk@contesting.com'
> > Subject: re: [TowerTalk] XM240 SWR plots
> >
> > Lossy traps may contribute to this exceptional bandwidth.
> >
> > John KK9A
> >
> > W7ZZ wrote:
> >
> > The XM240 will have an SWR of 2:1 or less over either the CW or SSB
> portion
> > of the band if tuned according to Cushcraft's dimensions. The "mid"
> setting
> > is intended to straddle the high end of the CW band and the low end of
> the
> > phone band. I have one at 85 feet, measured for the MID dimensions, and
> it
> > covers the SSB portion of the band beautifully but the SWR starts to rise
> > quickly below the phone band. Your mileage will vary due to height above
> > ground and local issues.
> >
> >
> >
> > 73, Doug W7ZZ
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
>
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