Hi Bill,
(If you don't mind, I have shared this to the reflector since I have rx’d other
similar questions)
I have lately been using 4NEC2 (free and very good program:
http://www.qsl.net/4nec2/ <http://www.qsl.net/4nec2/> )
My model is not a “standalone” 40m yagi, but includes the 80m Yagi on the same
boom. (the 40m Yagi shares the boom with the 80m Yagi)
However, I would suggest you build you own model anyway and play with the
Reflector tuning and observe the F/B, gain, impedance and BW. It is very
informative. You will also realize that when the peak F/B approaches the
resonance point the BW will get real narrow. Also, you will see that when the
reflector is tuned that short, the Yagi will actually reverse direction below
the FB peak since the reflector begins to act like a director. Thats why you
need bandswitching.
The most important thing about building your own model is you can then
determine the resonant point of the reflector when the driver is isolated. In
the model, insert about 1e6 ohms load at the driver element, and then do a
sweep and measure the resonance. Then, when you build the Yagi on the tower,
isolate the driver (just open it up) and use your analyzer to tune the
reflector for resonance at the measured qrg as per the model results. Then tune
the driver and include the necessary matching network based on the impedance
data gained from the model.
go here to my webpage…everything is explained for the coil build here:
http://www.qsl.net/ve6wz/CC_coil.html <http://www.qsl.net/ve6wz/CC_coil.html>
For detail on general coil construction and the tuning methods I used, see what
I did with the 80m Yagi here:
http://www.qsl.net/ve6wz/intro.htm <http://www.qsl.net/ve6wz/intro.htm>
73, de steve ve6wz
On Mar 28, 2018, at 4:25 PM, Bill Cotter <n4lg@qx.net> wrote:
>
> Hi Steve,
>
> I was intrigued with your note below on improving the XM240 antenna. I have
> an older Cushcraft 40-CD that I would like to improve the F/B and possibly
> the gain, but I only care about a bandwidth between 7000-7050kHz.
>
> I would like to know how you designed and built the Hi-Q coils to replace the
> stock coils. And, if you would share your design dimensions for maximum F/B,
> or the model file so I can recreate it down in the bottom CW segment. For
> tools, I have YO, AO and EZnec modeling software, plus a rig expert analyzer.
>
> I love this lightweight antenna and want to take it to the next step. BTW: I
> have toyed with the idea of building a W6NL Moxon for 40M, but I'll do that
> from scratch with more substantial materials. Getting this 40-CD back up is
> my short term goal.
>
> Thanks in advance,
>
> 73 Bill N4LG
>
>
> At 12:27 AM 3/28/2018, you 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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>
>
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