Let's consider that one again, Carl.
If we place a 4:1 balun at the feedpoint of the 80m antenna, then we have
625 Ohms "Unbalanced" at the feedpoint on 40m. the math is correct, but
normally we would attach the coax to the bottom side of the balun, not
openwire. The 625 ohms is a 12:1 SWR for the coax. So we don't want to do
that. And if we are going to run openwire, then we attach it directly to
the antenna, we don't use a balun at that point.
If we attach the openwire at the antenna and run it to the shack, into a 4:1
balun at the tuner, just what value the antenna system exhibits at that
point will depend on the length of the feedline. Granted it appears a 4:1
balun would be a better match and usually it would be from an impedance
standpoint. However if the SWR happens to be above 3:1 or so, the 4:1 balun
quickly loses its ability to impede common mode current and that is very
bad. The 1:1 current balun does not. Again as Jim says, we should be
calling it a choke, not a balun but old habits die hard. So as long as your
matchbox is capable of matching whatever mismatch there is in the antenna
system, it's better to use a 1:1 balun (for the sake of impeding CMC) and
let the matchbox do the matching.
The real problem is the other way around, when you attach openwire feedline
to a 40m dipole and attempt to match it on 80m.
Now I know they recommend not to do this, but I've had a few QTH's where it
was my only way of getting on 80m. I didn't have enough space for anything
else, so I ran this and worked lots of DX with it on 80. But it creates some
real challenges for the balun and the matchbox:
In this case the impedance is likely to be less than 10 Ohms at the
matchbox, but it also depends on the length of the feedline. Let's call it
12 Ohms for the sake of easy math. The 4:1 balun at that point transforms
it in the wrong direction, down to 3 Ohms. ALL matchboxes have high losses
when trying to deal with such low impedance. Just read any ARRL test of any
matchbox and check the internal loss at this impedance. Clearly the 12 Ohms
will be easier to match than 3 Ohms. AND the 1:1 balun/choke (assuming it
is adequate for the job) will do its job of impeding the common mode
current, whereas the 4:1 balun will fail miserably.
On top of that, the voltage balun fails in any case.
The 238 has the wrong type of balun (voltage) and the wrong ratio (4:1).
Don't use it.
Use an external 1:1 current balun capable of dealing with the power level
you are using.
Jim (K9YC) has written an excellent paper on this topic. It is well worth
reading.
73
Rick, DJ0IP
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Carl
Moreschi
Sent: Friday, July 12, 2013 4:56 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] OCF antennas evolution
Let's say you are running a 80 meter dipole on 40 meters. The impedance
will be about 2500 ohms or higher. Let's also say you are using 600 ohm
ladder line.
Then by placing a 4:1 balun at the antenna, the 2500 ohms becomes 625 ohms.
That means you have an excellent match on the 600 ohm ladder line. You then
match the 600 ohm line to the rig with a tuner.
The 4:1 balun in this case ends up with less loss than the 1:1 balun.
It all depends on what you are matching as to which one is better.
Carl Moreschi N4PY
121 Little Bell Dr.
Hays, NC 28635
www.n4py.com
On 7/12/2013 9:15 AM, Bob McGraw - K4TAX wrote:
> I commented recently where there is indeed a good application for 4:1
> balun usage. I think you have also indicated the correct use for 4:1
> baluns in some of your OFC antenna discussions. In most other cases,
> typically 1:1 ratios are preferable. I know that Jim , K9YC, has some
> good information and history on correct balun usage. To complicate the
> topic, there are current baluns and voltage baluns both in 1:1 and 4:1
> configurations as well as other ratio version. You are correct in that
> the topic is rather complex and there's widely varying opinions,
> approaches and results.
>
> In my case balun usage is in the 50 ohm to 50 ohm unbalanced to
> balanced configuration where a matched condition always exist. Thus
> being on the input of the tuner.
>
> 73
> Bob, K4TAX
>
>
> ----- Original Message ----- From: "Rick - DJ0IP / NJ0IP"
> <Rick@DJ0IP.de>
> To: "'Discussion of Ten-Tec Equipment'" <tentec@contesting.com>
> Sent: Friday, July 12, 2013 1:26 AM
> Subject: Re: [TenTec] OCF antennas evolution
>
>
>> Mike,
>>
>> I don't think anyone here is going to be able to explain to you in a
>> single email why the 1:1 balun is better.
>> However I can show you how to learn about this in a systematic way,
>> using material from 4 of the experts in this field.
>> It will probably take you about 10 hours to do this, but then you
>> will know.
>> (otherwise, just take our word for it.)
>>
>> This is a 4 part training course:
>>
>> PART-1: View GM3SEK's online presentation on common mode current
>> problems, especially the first half of it. View all of it but at this
>> point, you only need to understand what all can be causing us
>> problems. Ian's presentation is a good introduction to that. See:
>> http://www.ifwtech.co.uk/g3sek/in-prac/
>> (Note: it is a challenge just to bring the file up. You have to right
>> click Ian's link and save the link to your computer. Then
>> double-click the link on your computer and it brings the PowerPoint
>> file up.)
>>
>> PART-2: Read W7EL's excellent paper on "Baluns: What They Do and How
>> They Do It". These 8 pages will give you a good understanding of
>> Baluns (which Jim rightly called Chokes), and begin to answer your
question. See:
>> http://www.eznec.com/Amateur/Articles/Baluns.pdf
>>
>> PART-3: View G3TXQ's excellent page on "Tuner Balun Ratios":
>> http://www.karinya.net/g3txq/tuner_balun/
>>
>> PART-4: Now that you have a fairly good understanding of all of this,
>> Read K9YC's paper, "RFI-Ham". Although this is the most comprehensive
>> paper, I don't recommend beginning with this one. This is like the
>> graduate course.
>> It is 66 pages long, but it has everything in it. See:
>> http://audiosystemsgroup.com/RFI-Ham.pdf
>>
>> If you haven't a clue about all of this and view these resources in
>> this order, you will probably understand most of what Jim wrote,
>> though if you're like me, you'll have to read it two or three times.
>>
>> SHORTCUT for your question, MIKE: Go straight to PART-3.
>>
>> However I recommend anyone asking this type of question, take the
>> time to work through all of this. It will be very beneficial.
>>
>> 73
>> Rick, DJ0IP
>>
>>
>> -----Original Message-----
>> From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Mike
>> Bryce
>> Sent: Friday, July 12, 2013 3:51 AM
>> To: Discussion of Ten-Tec Equipment
>> Subject: Re: [TenTec] OCF antennas evolution
>>
>> I quite aware that open line is generally considered lost less feed line.
>>
>> What threw me was the use of a 1:1 balun instead of a 4:1.
>>
>> That's the head scratcher.
>>
>> Mike wb8vge
>>
>> Sent from my iPhone
>>
>> On Jul 11, 2013, at 9:32 PM, k6jek <k6jek@comcast.net> wrote:
>>
>>> Mike,
>>>
>>> You're matching the antenna system, the antenna and the feed line.
>>> You
>> just can't separate the two. When the antenna impedance at a
>> particular frequency is different from the feed line characteristic
>> impedance, the impedance on the line is different every place on the
>> line. You're matching whatever it happens to be at the shack end of
>> the feed line.
>>>
>>> Losses are the reason to put the balun near the station instead of
>>> near
>> the antenna. Open wire line has much lower losses than coax under
>> conditions of very high SWR. That's the reason we put up with the
>> stuff which is a royal pain in the arse, just so we can have a
>> ridiculous SWR and not care about it. And very high SWR is exactly
>> what we have at almost all frequencies when using a doublet as a
>> multi-band antenna. The only reason we can get away with such a thing
>> is the low loss of open wire line.
>> So you
>> want to run that stuff as far as you can before switching to coax. As
>> long as you can is ideally right into a balanced tuner, no balun at
>> all.
>>>
>>> Jon
>>>
>>>
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