On 6/30/2015 12:40 PM, Tom W3TDH wrote:
On Monday, June 29, 2015 at 8:46:47 PM UTC-4, Wayne wrote:
"Dave Platt" wrote in message ...

In article ,
Wayne wrote:
So, lets begin again, with no distractions.

What is the purpose (or benefit) of using a 1:4 unun on a 43 ft
vertical.

# http://www.eham.net/articles/21272 has a nice analysis.

# It looks to me as if:

# - Without a 4:1 unun, the antenna provides a very nice match at three
# frequencies with in the HF band. At other frequencies, the SWR is
# up over 10:1 much of the time - high enough that a coaxial feed
# can be rather lossy.

# - With a 4:1 unun, you do lose the excellent match at those three
# frequencies... but the match gets better at most other
# frequencies. The SWR across the HF band is much more uniform, and
# lower on average... low enough to cut the coax losses somewhat and
# (I think) within the matching range of many rigs' "line flattener"
# built-in autotuners.


Thanks Dave. I'll have to spend some more time studying it, but the article
is along the lines of what I was looking for.

I would assume that the 1:4 causes behavior just as you say....worse SWR at
nearly matched frequencies and better SWR elsewhere.

I'll have to pull out some textbooks and see how the math works out for a Z
seen through a 1:4 unun.

In practice, I've had good results with SWRs even in the 30:1 range with
short coax feeds.

More research...and thanks.

I know that what I am about to say is provocative to some but I still think it is worth saying. If you look at the way that commercial and military radios are matched to antennas you will notice that most of the matching is done as close to the feed point as practical.

Since only the power that actually reaches the antenna can be radiated I have a hard time seeing the point of matching the transmitter to the feed line. Matching at the feed line connection point will prevent damage to the transmitter but if that were the main objective a dummy load would accomplish that.

When you couple the antenna to the load at the feed point you can have extremely low losses in the feed line. When you do the matching at the feed point you will transfer the most energy possible to the antenna and will get the highest available effective radiated power. Since the objective is the transfer of the highest practical amount of power to the antenna the place to do that is at the feed point were possible.

I do realize that it is often simpler and easier to match at the feed line connection but I felt obliged to point out that is is not the most effective place to do the job.

Tom


Tom, very close.

Yes, it's most effective to match the feedline to the antenna at the
antenna connection. But it's also important to match the transmitter to
the feedline.

This latter piece is often ignored because people will use a feedline
who's characteristic impedance matches the transmitter already (i.e. 50
ohm line for a 50 ohm transmitter).

However, there are exceptions. For instance, if you're feeding a 75 ohm
antenna (i.e. a dipole) with 75 ohm coax, a 1:1 balun at the antenna
will provide a good match (ideally, 1:1). But there will be a 1.5:1
mismatch to a 50 ohm transmitter. In this case it would be better to
have the matching network at the transmitter.

You could also feed the antenna with 50 ohm feedline and place the
matching network at the antenna. The effect would still be a 1:1 SWR,
but the lower impedance of the coax would create higher i^2R losses; not
important if you're talking a short line, but a longer one would lower
output at the antenna.

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