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Author Topic: spiral antenna tuning  (Read 2104 times)
KB3TTP
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Posts: 32




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« on: April 15, 2010, 02:49:38 PM »

I am homebrewing an antenna similar to the multiband TAKTENNA.
I was just looking for some tips on the tuning of the antenna. Especially where the hookup wires are initially located. I have seen good and bad reviews of this antenna. I just want to expeirament and HAVE FUN!
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WB6BYU
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Posts: 13142




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« Reply #1 on: April 15, 2010, 03:05:26 PM »

The spiral coil is basically a loading inductor, perhaps partially acting as a capacitance hat
as well.  The required inductance depends on the amount of antenna past the coil, which
is somewhat difficult to define when the coil is at the end of the element:  the dressing
of the shorting wire (and whether it it attached to the inside or the outside of the coil)
makes a big difference in the actual setting.

But if you are trying to duplicate the TakTenna make sure you do NOT use a balun,
since radiation from the shield of the coax accounts for most of the signal from such
a short antenna.  And that is a big reason why the performance is so variable:  when
mounted high in the air you have a lot of coax radiating, while on a balcony it may only
be a few feet (but will almost always be more than the radiating conductor length of
the TakTenna itself.)
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WX7G
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Posts: 5972




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« Reply #2 on: April 15, 2010, 05:19:15 PM »

You can read an in-depth technical analysis of the Tak-tenna in AntenneX, the online antenna magazine. One test was with no feedline. The article appeared about 2-1/2 years ago and was written by me.

It is not just the coaxial feedline shield that becomes a radiator but the AC wiring in the house. In an apartment the Tak-tenna can excite common-mode RF current on the AC wiring of the entire apartment building. As antennas go that can be a good thing.

The Tak-tenna is still sitting in the garage.
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N3OX
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« Reply #3 on: April 15, 2010, 07:02:58 PM »

In an apartment the Tak-tenna can excite common-mode RF current on the AC wiring of the entire apartment building. As antennas go that can be a good thing.

Or a really terrible thing.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KB3TTP
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Posts: 32




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« Reply #4 on: April 15, 2010, 07:20:37 PM »

It's about what I thought. Maybe I will prevent it from staying in the garage, my basement, or my mailbox.

Thanks to all who responded
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WX7G
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« Reply #5 on: April 16, 2010, 08:16:59 AM »

Take a look at the eham reviews of the Tak-tenna and you will see that hams love it. It and the Isotron are the same thing - a short feedline current excitation device. I have tested both antennas and the Tak-tenna is easier to tune-up. An antenna analyzer makes for a quick job.
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N3OX
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« Reply #6 on: April 16, 2010, 08:34:52 AM »

Take a look at the eham reviews of the Tak-tenna and you will see that hams love it. It and the Isotron are the same thing - a short feedline current excitation device.

It's an interesting phenomenon.  If you sold a simple end loaded antenna with a spiral coil out on one end and a feedpoint at the near end, everyone would obsess about a need for radials and never buy it.

You build the same antenna but move the feedpoint up to the hat end, and everyone buys it and reviews it well because it's being sold as a self-contained antenna even though it needs radials down at the bottom of the coax exactly as much as the former version.

I love how a lot of the people who are happy with 40m versions of this type of antenna have it mounted on top of a 20-30 foot mast :-)    Probably makes it as good as any other 40m slightly loaded vertical that's mounted over a bad ground system... which, for someone who thinks they can't have an antenna, is quite good.

But if they went a little further and laid radials in the yard and just used the 20 foot stick as a 40m vertical, it would work GREAT.

Ah well, fun to play with it and learn what it does.  Just don't forget the current meter on the feedline as part of the experimenting ;-)

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KB1GMX
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Posts: 760




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« Reply #7 on: April 17, 2010, 09:57:58 AM »

Another way you can greatly improve the antenna..

the center section is a mere 3ft if I recall.  Build it with a 10ft center section.  Less spiral needed and
still small but at 40M it works a whole loads better.

It's no different from any other loaded antenna, short
is inefficient but, that may be handy if space is an issue.  The point being if you can make it bigger and
at that size even a little makes a difference.

Allison
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KB3TTP
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Posts: 32




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« Reply #8 on: April 17, 2010, 10:19:03 AM »

This is all very interesting. Something to play with I'm sure. I was think of making the spirals much larger, but then I would be defeating my own purpose being to expierement.

Hmmmmm. Sounds like I'm getting to the size of a Pfeiffer maltese quad. Which already seems to be proven.

73
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WX7G
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Posts: 5972




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« Reply #9 on: April 17, 2010, 11:15:03 AM »

KB1GMX, but the Tak-tenna is not a short antenna. The tuning procedure guarantees that the shield side of the antenna does nothing. It is not a dipole composed of the two spiral halves but an antenna composed of one half and the coax shield.

I did tune a 40 meter Tak-tenna as a short dipole. To do that the feedline was eliminated and a beta match was used to match the low input impedance to 50 ohms. It worked as modeled with a radiation efficiency of a few percent and narrow bandwidth.

N3OX you are right. Forget putting the Tak-tenna at the top of a 30' mast. Feed the mast instead.

I've analyzed small antennas for years and have written many article about them. Dave's rules of small antennas:

SMALL
EFFICIENT
WIDE BAND

Pick any two
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WX7G
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Posts: 5972




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« Reply #10 on: April 17, 2010, 11:26:02 AM »

If properly constructed a 10' 7 MHz dipole can be 50% efficient. See AntenneX about 2 years ago for a 5' 14 MHz dipole with such efficiency and a very easy tuning procedure. It uses a 'tuning bar.'

The top hats are large. 3' diameter for 14 MHz and 6' for 7 MHz. That makes the Tak-tenna look tiny but it is a true dipole as long as the feedline is decoupled.
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