[TowerTalk] Auto tuners, Baluns and open wire feeders

[jimlux]

On 7/17/18 2:34 PM, Kim Elmore wrote:
> I’ll display my benighted state by asking a question: why not place an auto-tuner at the feed point of the doublet? That way, the feed line is flat and the doublet can be any length, assuming impedance say desired frequencies are in tang of the tuner. Then, add common mode chokes at the  auto-tuner input and anywhere else desired. Won’t that do the job?
> 
> Kim N5OP
> 

That *is* a good way to do it.  There are some potential practical problems:
1) autotuner is heavy
2) so you'll mount it to some support. There's some TBD coupling between 
the tuner case or innards and the support.  whether that causes a 
problem is kind of hard to tell.


But "tuner at feedpoint" is probably the best overall solution.

It also kind of depends on what you're trying to do.  Recently, I've 
been working with a HF receive antenna for 5-30 MHz that's a dipole 6m 
long.  It turns out that if I use a 16:1 transformer into a 50 ohm 
receiver, my overall system response roughly matches the curve from the 
galactic background noise.  Basically, I'm presenting a 6 meter dipole 
with an 800 ohm load.

Since Galactic background rises at lower frequencies (down to around 4-5 
MHz), the fact that the antenna is "shorter and shorter" and gets less 
sensitive just about balances out (within 1 dB).  The amplifier noise is 
about 4-5 dB lower than the Galactic background, so with a pretty simple 
system, I'm "Galactic noise limited"

So, no tuner required, no real matching network required, etc.
Would I use it for transmitting? Not a chance.  But for receive, its 
hard to think of a simpler design (other than maybe an op amp with a 
High Z load resistor on the antenna).

(For HF frequencies the galactic noise (curve D) lines up with the curve 
C for "rural quiet"..  Curve E for urban is about 20dB higher.)







> "People that make music together cannot be enemies, at least as long as the music lasts." -- Paul Hindemith
> 
>> On Jul 17, 2018, at 15:19, Wes Stewart <wes_n7ws at triconet.org> wrote:
>>
>> The man wants a simple, single wire antenna.  How the heck is choking at the input end of a short length of open wire significantly better that doing it at the feedpoint?
>>
>> I know you and Dean are buddies, but don't get me started about him.  Since he's using his software which has been shown to be in error in the past, "Introducing an Improved Version of Transmission Line for Windows Software"  QST, Jun 2014, p.39, I wouldn't make bank on the conclusions.  Plus he continues the fallacy that a balun on the input of a tuner is better than one on the output.  Shown to be wrong by W7EL, W8JI, W9CF among others. ARRL's publication of such an unbalanced tuner and pretending it's balanced is a disservice to the community.
>>
>> I agree with your recommendation of fan dipoles, but that is beyond the scope of the OP's desires.
>>
>> Wes  N7WS
>>
>>> On 7/17/2018 11:32 AM, Jim Brown wrote:
>>>> On 7/17/2018 11:00 AM, Wes Stewart wrote:
>>>> If your open wire is short, why bother? Replace it with low loss coax, put a decent common-mode choke at the feedpoint and tune it.  Sometimes the effort to use open wire simply isn't worth it.
>>>
>>> Caution on this. See JUN 2015 - QST (PG. 30) "Don't Blow Up Your Balun"
>>> by Dean Straw, N6BV
>>>
>>> Dean's work clearly shows why it simply is not practical to choke a non-resonant antenna. And, because receive noise is such a major issue for most of us, it's why I don't recommend antennas like this. The random wire and off-center fed wires were great ideas for 30 years ago, but terrible ideas today.
>>>
>>> Fan dipoles are a FAR better choice.
>>>
>>> 73, Jim K9YC
>>>
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