[TowerTalk] XM240 SWR plots

[VE6WZ Steve]

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 at kk9a.com> <john at kk9a.com> wrote:
> 
> I should have said lossy loading coils may contribute to this exceptional
> bandwidth.
> 
> John KK9A
> 
> -----Original Message-----
> From: john at kk9a.com [mailto:john at kk9a.com] 
> Sent: Wednesday, March 28, 2018 01:24
> To: 'towertalk at 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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