[Elecraft] 135ft flat-top with K2, KAT2, BL2, and 450 ohm feedline

[Jack Brindle]

Don suggested this would be worth posting...

When dealing with antennas, things are never as "precise" as we might  
want them to be. Folks talk about antenna tuners as if they work  
magic, then wonder why things aren't as good as they should be. An  
ATU at the shack in line with a coax-fed antenna has one big purpose.  
It provides a nice load for the transceiver. It does absolutely  
nothing to help out the feedline or antenna itself. The very precious  
RF gets burnt up in the coax (with its unknown, but potentially very  
high SWR) between the tuner and the antenna. There are strange RF  
currents all over the place, especially on the coax, which not only  
heats the coax, but tend to screw up the antenna radiation pattern as  
well.

My belief is that there are three solutions to the problem.
1) Place the ATU _AT_ the antenna feed point. The problem is that it  
is really a pain to do in most cases.

2) Use matched antennas on all bands. Cebik points this out in a  
great paper on his website that discusses open-wire vs coax fed  
antennas (see "To Trap or Not To Trap",
http://www.cebik.com/wire/trapqq.html). This is also the solution  
used by the larger (more serious, lots of money to spend) contesters.

3) Use open-wire feed. There we don't care about the mismatch since  
the feedline loss is microscopic. The RF currents balance themselves  
out so that the feedline doesn't radiate. But we then need a decent  
tuner to feed the antenna & open-wire. Here I drop back to the old  
tank-circuit tuners which make things "sing!" I don't like to use a  
balun here - it's just another device to burn up the precious RF when  
you hit a severe mis-match node.

Now if you don't have a swinging-link balanced tuner, but do have an  
L-network tuner and are running low power, go ahead with a 1:1 balun.  
At low power there is little chance you will saturate the balun core  
unless you get really unlucky and hit a severe node at the operating  
frequency.

I do use coax to feed my antennas, but only for those that are  
matched for the band/frequency. For all others, especially ones that  
are used on multiple bands, I use open-wire feed. The length of the  
feed really doesn't matter because of the super low loss. What  
happens is the feedline transposes the antenna feed impedance to a  
value dependent on the feedline length and operating frequency. The  
balanced antenna tuner then matches whatever impedance it is  
presented with, making the transceiver very happy. And with the low- 
loss characteristics of open-wire feed, just about everything the  
K2/100 puts out gets radiated by the antenna!

On Oct 1, 2006, at 6:20 PM, Don Wilhelm wrote:

> Zac,
>
> Why guess?
>
> You can find the antenna feedpoint impedance for various bands on L B
> Cebik's website (at least it used to be there) www.cebik.com .
> Armed with that information along with the known length of your  
> feedline,
> use TLW which you can download from the ARRL website
> http://www.arrl.org/notes/9043/TLW3B.zip and feed the information  
> into TLW
> to find your shack end impedance for each band of interest.  A bit  
> of cut
> 'n' try with TLW can find you the best compromise feedline length  
> to ues
> with your antenna.
>
> 73,
> Don W3FPR
>
>
>> -----Original Message-----
>>
>>     I'm sorry but I need to add this before Zac Brown does something
>> real stupid. When using 450 ohm feed line you must use this EXACT
>> length. The exact length is that which goes from the antenna to the
>> balun in your shack.
>>
>>     Any talk about matching the 450 ohms is not correct. The feed  
>> line
>> runs with a very high SWR and since it is low loss there is just a  
>> tiny
>> loss. It will correct to a 1:1 SWR between your tuner and the radio.
>>
>>     And about the balun. Seems if you have a 4:1 or 1:1 to choose  
>> from I
>> would choose the 1:1 because it does a better job of keeping the  
>> current
>> balanced in the balanced feed line.
>>
>>     This is pretty complex stuff but you Zac do not need to do the  
>> math.
>> Just put it up and use it. I have been using it for 8 years now.  
>> Had to
>> get feed line. A hail storm just beat up my feed line :-)
>>
>>     I have done the math. It takes several Smith Charts to plot the
>> current through the system.
>>
>> 73 Karl K5DI
>>
>>
>>
>>
>> Zac Brown wrote:
>>> So what's the best way to add some more line to the middle of my
>>> feedline?  Is there a technique for joining two pieces of 450 ohm
>>> ladder line that doesn't create an imbalance in the line?  Can I  
>>> just
>>> leave one "leg" straight and then wrap the corresponding  
>>> "leg" (on the
>>> other piece) other around it, then solder them together?  Or  
>>> should I
>>> just leave both legs straight and then solder them together?  Or how
>>> about using those twist locks that you use for wiring up lights?
>>>
>>> This is probably a silly question, but I took great care to route  
>>> the
>>> line away from anything that would imbalance it, and would hate to
>>> mess that up now.  This is my first experience with balanced line :)
>>>
>>> Thanks,
>>>
>>> Zac
>>> KD5IEF
>>> K2# 4907
>>
>
> _______________________________________________
> Elecraft mailing list
> Post to: Elecraft at mailman.qth.net
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

- Jack Brindle, W6FB
------------------------------------------------------------------------ 
---------------------