[TowerTalk] Broadbanding 80 meter dipole

[Jim Thomson]

Date: Fri, 18 Aug 2017 07:34:43 -0700
From: Grant Saviers <grants2 at pacbell.net>
To: "Roger (K8RI) on TT" <K8RI-on-TowerTalk at tm.net>,
"towertalk at contesting.com" <towertalk at contesting.com>
Subject: Re: [TowerTalk] Broadbanding 80 meter dipole

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

##  That link is excellent.  How the heck I missed this one is beyond me.   I hope roger has a real good look at it.
It states that the concept can also be use for a 80 + 40m  fan dipole.  The swr broad banding will only work on 80m though,  not
on 40m, which is a moot point,  since any sloping 40M  dipole, using big gauge wire, like say  10 or 12 gauge,
will be broad anyway..on 40M. 

##  Since the simple broad banding trick is a mono band device, on paper, it should also work on any monoband antenna, 
like a monoband vertical...or monoband yagi  etc ????   

##  Rogers proposed  5 x half wave slopers is in the old arrl ant books.  Dead simple for the most part.  20 DB FB,and
a bit of gain.   But in that config, all 5 x feedlines were, I think, aprx  .375 wave long.  The idea was, the 4 un – used feedlines
looked like XL  at the feedpoint, making them into Reflectors.   If that is the case, the above broad banding technique may be 
a bit trickier to implement. 

##  On a half wave sloping dipole...hanging off a tower,  is a CM choke even required at the feedpoint ??   My guess is the half wave
sloping dipole would be un balanced...like a vert.   If Roger is planning to use the  5 x sloper array, the relay box in that config is isolated,
at least the relays are.   The un used slopers  have their braids floating inside the relay box.. via the relays used.   At most, a single CM  choke
could be used on the main 50 ohm coax, that feeds the input to the relay box...right at the input to the box. 

##  If done right, he could have his cake..and eat it too. 

Jim   VE7RF