[TowerTalk] 160-m. Inverted L Question
jim at audiosystemsgroup.com
Wed Dec 12 19:35:23 EST 2007
On Wed, 12 Dec 2007 19:18:03 EST, RLVZ at aol.com wrote:
>1) Some people report that the 160-m. Inverted L can be fed directly with 50
>ohm coax with good results. Is that true?
Yes, if use you RG8, it isn't too long, and have a decent tuner between the
coax and the transmitter. I've done that for more than a year, with decent
results. The TenTec 229 and 238 series of tuners (electrically identical) work
well for this application. My coax is about 35 ft long. This is a pretty small
mismatch, so 3-4 times that length could be tolerated.
>2) Some people feed their Inverted L's with fixed 500 or 1,000 pF capacitors.
>What advantages does this offer, such as: lower SWR at resonance, and/or
>increased bandwidth? (again these are "fixed" capacitors not variable),
To do this, you make the antenna a bit longer than a quarter wave so that the
Z at your operating frequency is something like 50 + j 40 ohms, then add a cap
in series where its reactance is 40 ohms. That makes it a perfect match for 50
ohm coax. I've done that too, and it works very well.
>3) Can you offer a good resource for the 500 or 1,000pF capacitors that will
>handle a Kw?
As it happens, I asked exactly that question several weeks ago. The "readers'
digest is that dipped micas with a sufficiently high voltage rating are the
weapon of choice. I needed .0022 uF to get 40 ohms. I first tried two 0.0047
uF 600V caps in series. They provided the match, but they got hot with my
Titan amp at 1.5 kW. I next tried the series/parallel of 8 0.0047 uF caps to
get the same value, and ran them for the ARRL 160 contest. They ran cool as a
cucumber. Careful signal reports from K6MM (ground wave, daytime, about 20
miles away) were that this arrangement was a fractional S-unit better than the
tuner only. (The tuner was still in the circuit, but doing far less matching).
The value of cap you need will depend on YOUR antenna, and what the inductive
X is when the R is 50 ohms. Use your antenna Z bridge to find that out. Or use
NEC to predict it. Or do both. :)
Jim Brown K9YC
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