[TowerTalk] Broadbanding 80 meter dipole

Sat Aug 19 15:08:46 EDT 2017

Thanks for that link Grant. I think I'll try it.  I have a couple of 
spools of RG8X-LL which will handle the legal limit if the SWR is low.
I tune the amp into a dummy load, so I want an SWR limit of 1.5:1 so it 
requires no tuning when switching to the antenna.  I currently use a 
line tuner on 40 as the SWR is low across (roughly) the bottom 3/4 of 
the band at the antenna. I typically operate below 7.2 and 3.8.  I may 
still need to resort to remote switching to keep the dual band 
capability.  If I can't then I won't be able to go with the 5 switched 
slopers.

The actual SWR (on 40) at the feed point is low enough that the 8X-LL 
easily handles the legal limit.  Not so on 75 where I've used either 
LMR400, or BuryFlex with one common mode choke at the antenna and 
another at the tower. I'm going to wind a dual choke with 10T of RG 400 
on 2 cores and 7T on another 2 cores in series at the feed point in 
hopes of negating the need for the current choke back at the tower.  A 
75 meter, center fed, sloping, half wave dipole from roughly 90' to 10' 
is far from being balanced. I'd like to use the RG400 for feedline, but 
that stuff is expensive!

I'm currently using LMR-400 for feedline with the chokes wound as part 
of the feed line, but will go to 8X-LL if I can broadband the 75 meter 
section.  Even as shown in the article, I can use the line tuner so the 
rig, or amp sees 50 ohms, simplifying the amp tuning.

Using 8X-LL would reduce both the weight and wind resistance substantially.

73, Roger (K8RI)

On 8/18/2017 Friday 10:34 AM, Grant Saviers wrote:
> The ARRL Antenna Handbook has a number ways to broadband a dipole, 
> several probably difficult as noted.
>
> One not in the current handbook is the Witt/Leeson tuned coaxial 
> resonator feed method,  see 
> http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1993/09/page27/index.html
>
> At QRO on 80m, small coax such a Davis RG8X-LL will be ok and RG6 is 
> QRO capable for the 75 ohm section per W8JI evaluations.  There are no 
> L, C or relays in the air, it's a plain wire dipole and the coax is 
> needed anyway to feed it, so it is cheap, light, and easy to maintain.
>
> I've built one with RG8X-LL and Belden 8281 double shield video cable 
> (often cheap on ebay) for next year Field Day but not run the in the 
> air swr curves yet.   As with any dipole there is height sensitivity 
> so for this application modeling with the tower is a good idea.  I 
> modeled it for my application with EZNEC/4 and optimized it with 
> AutoEZ and the swr is < 1.6:1 from 3.5 to 3.92Mhz, rising to 2:1 at 
> 4.0MHz.
>
> Grant KZ1W
>
> On 8/18/2017 0:56 AM, Roger (K8RI) on TT wrote:
>> I'd need 5 auto tuners and the configuration would require that the 
>> be close to in sync.
>> These are center fed, half wave dipoles.  I've had excellent results 
>> on 40 and 75.  Unfortunately with the common mode choke and BuryFlex 
>> hanging from the center. It requires a LOT of tension to keep the 
>> antennas straight...About 200# worth. There's a lot of 5/16ths double 
>> braid rope already on the tower through pulleys, making working on 
>> the antennas easy, but I don't think the 5 auto tuners would be 
>> practical.
>>
>> 73, Roger (K8RI) .
>>
>> On 8/17/2017 Thursday 3:20 PM, Rod Greene wrote:
>>> Or use an auto tuner at the feedpoint to match the slopers on 
>>> whatever band/freq you want to use. That's what I do.
>>>
>>> 73, Rod/w7zrc
>>>
>>>
>>> ------------------------------------------------------------------------ 
>>>
>>> *From:* Roger (K8RI) on TT <K8RI-on-TowerTalk at tm.net>
>>> *To:* "towertalk at contesting.com" <towertalk at contesting.com>
>>> *Sent:* Thursday, August 17, 2017 12:33 PM
>>> *Subject:* [TowerTalk] Broadbanding 80 meter dipole
>>>
>>> I've seen a few possibilities in the past, but I'd like to add a dew
>>> slopers to come up with the 5 used for some directivity, gain and front
>>> to back. Switched coils at the feedpoint with frequency sensing would
>>> likely work.  I've had excellent results with just a couple switched
>>> slopers on 40.
>>>
>>> To complicate things I'd like to do this and maintain the dual band 
>>> 75 /
>>> 40 meter operation, which would likely require separate feedlines. The
>>> unused line would / could end up as a stub.  I could possibly use the
>>> frequency sensing to disconnect the coax for the unused band.
>>>
>>> Thoughts?
>>>
>>> -- 
>>>
>>> 73
>>>
>>> Roger (K8RI)
>>>
>>>
>>> ---
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>>>
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>>
>
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-- 

73

Roger (K8RI)