[Antennas] From transmission lines to tuners

2002年12月21日 05:47:51 -0500

Kees and All,
I designed a 4:1 low (100 watts) power current balun which the NJQRP Club 
is offering as a kit:
http://www.njqrp.org/balun/index.html
I very carefully measured its losses accurate to 1% which are:
At 200 Ohms, 1:1 SWR, loss is 3%, or 0.12 dB.
At 714 Ohms reactive, SWR 6.6, loss is 6% or 0.27dB.
The balun is flat from 700 kHz through 30 Mhz. What you say about a balun 
being good for 10 meters but not 160 or 160 but not 10, I found to be true 
for voltage baluns. I connected one of the tordial coils of this balun as 
a voltage balun (it uses two), and it had the same low frequency coverage 
but SWR increased above 19 mHz. If you removed turns, the voltage balun 
high frequency coverage would increase at the expense of low frequency 
coverage. This indicates the principles of operation of the voltage balun 
vs a current are the limiting factor on frequency coverage, rather than the 
material. The voltage balun uses a phase shift in a transmission line 
wound on the tordial core. As the frequency becomes higher, the phase 
shift becomes inadequate to sustain the voltage ratio. A 1:1 voltage balun 
suffers from limited frequency range too. There's a lot more, which is 
covered in Sevick's books and summarized the kit documentation.
If you want more power or do not want to build a kit, I found the 
performance of the Radio Works B4-2KX current balun to similar but it will 
take 2 KW instead of 100 watts, and it costs more.
Actually, Sevick designed a Voltage Balun for use in an antenna tuner from 
160 through 10 meters for maximum power which he documents in his book 
"Building and Using Baluns and Ununs", but he used a very large torodial 
core, large wire and high voltage insulation.
LDG is offering a voltage balun kit for use with their antenna tuners, but 
I don't know anything about its performance.
At 09:30 AM 12/19/2002 -0600, Sandy and Kees Talen wrote:
>Since it's really an antenna "system", as has been pointed out by
>several, here is a highly recommended article on antenna tuners
>and their losses. The only thing I walk away with is "where do I
>find the parts to build a Johnson Matchbox equivalent". The next
>thing is how to test your beam (traps/joints, etc) on ALL the
>elements (and not with an ohm meter). Maybe this has been
>covered earlier.
>>Frank Witt's (AI1H) articles in 1996 QST are most informative
>and certainly indicate you can quickly loose more at the tuner
>than at the transmission line. It also makes a very good case for
>good link coupled tuners like the Johnson Matchbox vs the
>many, more compact, tuners we see today. I can see many of the
>old timers nodding their heads.
>>I gather from it, that typical toroid 4:1 balun losses for approx
>1.5:1 or better SWR are on the order of 0.5dB. Higher losses
>on the lower 80m band (1dB ....worse on 160m) are due to
>toroid material selection limitations (selected for good 10m,
>15m performance, can't cover 10m-160m with one type).
>>On the Heathkit tuner (similar to many being used today), I can
>see why lowest balanced line losses are observed using a 200
>ohm load with a 1:1 SWR due to the 4:1 toroid balun. Why are
>the lowest unbalanced line losses observed using a 200 ohm
>load with a 4:1 SWR ? A 50 ohm load with a 1:1 SWR is also
>low, but the 200 ohm load is lower according to his data ??
>>73 Kees K5BCQ

73, Chas, W1CG
K2 #462