Good morning all,
It appears that I'm not alone with this issue. It's a great antenna until you
try to push up the band for the digital modes, with potential for more to come.
So far I've learned this from this list and other friends in the VHF spectrum:
The theory behind moving the shorting bars out to more the freq up the band may
be based on simple J pole type design, when you short higher up the driven
element, it increases in resonate freq.
The 70 ohm (RG-6) phasing line might be able to raise the center freq if
shortened. If anyone has done this, please reply with length to attempt.
The driven element might be able to be sleeved increasing the diameter and
potential bandwidth, which is the biggest problem with this design.
Electrically lengthening the T match bars using retaining rings at the end may
create an aching point at hi power and wet environment. This may be
catastrophic for an amp and will not be explored.
Repositioning D1 may help but this has unknown effect of performance.
Finding a better, free space test range will be first on the list. I have 90Ft
trees here so spanning 2 trees and hanging the antenna in the middle show be
effective.
I still plan on calling M2 on Monday to see if I'm missing something
I will say this, great responses from the VHF community. Thanks to all.
73,
Paul Rollinson, KE1LI
Rollinson Associates
860-928-5147
860-208-8814 Cell/Text
Paulrollinson@sbcglobal.net
Representing Component Part
Manufacturers for over 35 years
-----Original Message-----
From: Jim Worsham [mailto:wa4kxy@bellsouth.net]
Sent: Saturday, January 2, 2021 11:30 AM
To: Paul Rollinson
Cc: VHF Contesting Reflector
Subject: Re: [VHFcontesting] Tech Help needed
Hello Paul. I had the exact same problem last year when I was setting up to
operate from home in the June and September contests. I also have a 6M7 and
after a weekend of trying I was also unable to tune it high enough in the band
to get an SWR less than 2 at 50.318 to make the solid state amplifier happy.
Like you the shorting bars seem to have little or no effect. It seems like the
M2 antennas of that time were built to operate at 50.100 plus or minus 100 kHz
and that was it. Luckily I had another 6M antenna available that I was able to
tune so I put it up instead. The 6M7 sits in my basement now unused. I would
really like to hear what the result of your conversation with M2 is. Thanks.
73
Jim, W4KXY
> On Jan 2, 2021, at 9:02 AM, Paul Rollinson <Paulrollinson@sbcglobal.net>
> wrote:
>
> Good morning all,
>
>
>
> I hope you had a nice holiday.
>
>
>
> Part of prepping for the January contest included taking down the
> stack ofM2 6M7antennas and tuning them a bit higher to accommodate
> FT8/4 frequencies where the swr runs close to 2:1
>
>
>
> After several hours of tweaking we were unsuccessful in elevating the
> frequencies. Moving the shorting bars had little effect and the
> elements are crimped together so shortening them would be permanent; I
> prefer not to do this.
>
>
>
> After diving thru the specs, the antennas are 6M7 and not the modern
> versions 6M7JHV. Below is a link to the manual.
>
> https://dl-mail.ymail.com/ws/download/mailboxes/@.id==VjN-j4MNikUvGG6d
> I72pGt
> Fnbtnm78WZEHQ84kCP5dKOEa_RWizZIRV1VtWw7y3GBuOyPtJBEyjJRlraKdU25EkUFw/m
> essage
> s/@.id==AOqprj4WDiqnX--apQiykLPEILk/content/parts/@.id==2/raw?appid=YM
> ailNor
> rin
> <https://dl-mail.ymail.com/ws/download/mailboxes/@.id==VjN-j4MNikUvGG6
> dI72pG
> tFnbtnm78WZEHQ84kCP5dKOEa_RWizZIRV1VtWw7y3GBuOyPtJBEyjJRlraKdU25EkUFw/
> messag
> es/@.id==AOqprj4WDiqnX--apQiykLPEILk/content/parts/@.id==2/raw?appid=Y
> MailNo
> rrin&ymreqid=117033b5-9242-646b-1c92-e7000301a500&token=zitEzqOML3j84e
> 6ealFT
> T5U7-km5qEQF52lp7AcCuBZYNpCJiKFonMipX0nRikauD7BKLkpZAtQ_aN6eUBTbi2UcMC
> 60Zo-i
> fhoLye8kBZhFaDq07_sCfvo7gxJ36C_A>
> &ymreqid=117033b5-9242-646b-1c92-e7000301a500&token=zitEzqOML3j84e6eal
> FTT5U7
> -km5qEQF52lp7AcCuBZYNpCJiKFonMipX0nRikauD7BKLkpZAtQ_aN6eUBTbi2UcMC60Zo
> -ifhoL
> ye8kBZhFaDq07_sCfvo7gxJ36C_A
>
>
>
> I'm looking for advice. The manual states that moving the shoring bars
> out will increase the tuned frequency (which doesn't make sense to
> me). If this is true, I may experiment with either adding push on
> rings to act as capacitance hats, electrically lengthening the t
> match, or swedge on lengths of rod to extend the t matches.
>
>
>
> The design freq of the antenna is very sharp: 49.5 to 50.5MHZ but I
> can't seem to stay within 2:1 at 50.315. Running 1 kw is an issue for the amp.
>
>
>
> The other, not preferred option is to run a short segment of 70 ohm
> coax as a transformer but I will lose output/receive in the form of heat.
>
>
>
> We hung the antennas about 10 ft in the air to sweep so the numbers
> are fairly accurate.
>
>
>
> Any help or suggestions would be appreciated.
>
>
>
> Thanks,
>
>
>
> Paul Rollinson, KE1LI
>
> FN41AU
>
> 860-928-5147
>
> 860-208-8814 Cell/Text
>
> <mailto:Paulrollinson@sbcglobal.net> Paulrollinson@sbcglobal.net
>
>
>
>
>
> Happy Holidays
>
>
>
>
>
> --
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