[VHFcontesting] Tech Help needed

[David Olean]

I think the problem is that the Q of the antenna is so high that the 
elements cannot supply a reasonable reactance at the FT-8 frequency for 
the matching section to work with. The solution is to try to tweak the 
driven element or maybe director #1 to raise frequency a bit.  After 
all, if the elements are not resonant, then no amount of adjustment of 
the T match or any other type of matching system, will have any effect. 
I have not played with a 6M7, but do note that director #1 is quite 
important in coupling into a resonant set of elements.  As you get 
farther away from the driven element, the element dimensions are less 
important.One way to try a change is to drill and tap the element end so 
that you can add more metal if trimming the element shorter does not 
give you what you want. A small machine screw can add element length.

Dave K1WHS

On 1/2/2021 9:48 AM, peter h via VHFcontesting wrote:
> 0645 pst 02jan2021
> Perhaps contacting M2 directly, with a cc to K6MYC might provide the needed info.
> bt73Pete, N6ZE in DM04ne with an older 6M7 also
>
>
> -----Original Message-----
> From: Paul Rollinson <Paulrollinson at sbcglobal.net>
> To: 'VHF Contesting Reflector' <vhfcontesting at contesting.com>
> Sent: Sat, Jan 2, 2021 7:02 am
> Subject: [VHFcontesting] Tech Help needed
>
> 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-j4MNikUvGG6dI72pGt
> Fnbtnm78WZEHQ84kCP5dKOEa_RWizZIRV1VtWw7y3GBuOyPtJBEyjJRlraKdU25EkUFw/message
> s/@.id==AOqprj4WDiqnX--apQiykLPEILk/content/parts/@.id==2/raw?appid=YMailNor
> rin
> <https://dl-mail.ymail.com/ws/download/mailboxes/@.id==VjN-j4MNikUvGG6dI72pG
> tFnbtnm78WZEHQ84kCP5dKOEa_RWizZIRV1VtWw7y3GBuOyPtJBEyjJRlraKdU25EkUFw/messag
> es/@.id==AOqprj4WDiqnX--apQiykLPEILk/content/parts/@.id==2/raw?appid=YMailNo
> rrin&ymreqid=117033b5-9242-646b-1c92-e7000301a500&token=zitEzqOML3j84e6ealFT
> T5U7-km5qEQF52lp7AcCuBZYNpCJiKFonMipX0nRikauD7BKLkpZAtQ_aN6eUBTbi2UcMC60Zo-i
> fhoLye8kBZhFaDq07_sCfvo7gxJ36C_A>
> &ymreqid=117033b5-9242-646b-1c92-e7000301a500&token=zitEzqOML3j84e6ealFTT5U7
> -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 at sbcglobal.net> Paulrollinson at sbcglobal.net
>
>   
>
>   
>
>                    Happy Holidays
>
>   
>
>
>