Interesting guys. I have had up a full size Telrex monobander (2 el) since
1995. It has 66 ft full size elements on a 14 foot boom. It has been a killer
since it went up. One of the best investments I have ever made. It is 6 ft
above an M2 rotary for 80/75 at 112 ft.
73,
John, W4NU
K4JAG 1959 to 1998
Sent from my iPhone
> On Mar 28, 2018, at 8:29 PM, Steve Babcock <ve6wz@shaw.ca> wrote:
>
> 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
>>>>
>>>> _______________________________________________
>>>>
>>>>
>>>>
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>>> _______________________________________________
>>>
>>>
>>>
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>>
>>
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>
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