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
>
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>
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