On 11/23/2014 12:53 PM, Wayne wrote:


"John S" wrote in message ...

On 11/23/2014 11:17 AM, Wayne wrote:


"John S" wrote in message ...

I like antennas that are short-circuited DC-wise. Mainly because, at
one time I had a radio on a vertical and lightning struck about a mile
away and blocked my receiver for several seconds. Of course, not all
radios have a floating input, but, it impressed me.

So, I "invented" the folded unipole which would match to a 50 ohm feed.

Any interest?

Yes.

# By the way, Wayne, my interests in antennas have recently been in the
# 70cm band (specifically 434MHz) where there are numerous cheap modules
# to play with. They have only 1 or so mW output and are illegal in the US
# for unlicensed operation except under strict limitations. However, with
# a license, I can do what I wish as long as I comply with the
# regulations. My software always sends my call letters at each
# transmission, for example.

# As for the folded unipole, first imagine a normal unipole. Remember that
# the feed resistance is about 30 or so ohms, yes? So, the recommendation
# is to droop the radials to about -45 degrees. This raises the feed
# resistance to about 50 ohms.

# Now, the folded unipole has a higher feed resistance than we want.
# Hmmmm.. what if we RAISE the radials to compensate? Yeah, stupid, right?

# It works, and I have a model here which exhibits SWR so low I cannot
# measure it except with a vector voltmeter.

# I am not recommending this for the average amateur because tuning the
# antenna is a bit troublesome for many. It is a bit easier to adjust a
# normal unipole's length than a folded unipole's length. That's why I
# haven't pushed it.

Interesting.
But, I would expect the folded unipole to have a feedpoint of about 150
ish ohms over perfect ground.

Yes, approximately, but using 4 radials rather than perfect ground. Feed
point resistance approximately 150 ohms as you say.

Is that the antenna that you have built and tested?

No. Picture the folded unipole with the feedpoint resistance you imagine
with 4 radials. Now, raise the angle of the radials about 23 or 25
degrees to get the feed point resistance desired.

Remember that the typical ground plane needs a droop of about 45 degrees
to raise its feed point resistance, so we are doing just the opposite
here. It will lower the feed point resistance.

I have an EZNEC file you may have, when you are ready.