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Author Topic: Attic Slinky - Sharp Tuner Settings?  (Read 4752 times)

Posts: 243

« on: March 08, 2011, 12:41:17 PM »

I have a slinky dipole in the attic about 25ft long and about 30ft above ground level. It seems to work well but no reference for comparison.  Using a Dentron Super Tuner connected to open wire (ladder-line) via its internal balun. I am able to get resonance on most bands. However, on some the resonance point is very sharp/narrow. So, my question is with a very narrow setting on the tuner, for close to 1:1 SWR, is that bad or good? My non-professional guess is that I have a hi-Q situation with the feedline and the antenna.  Huh

The Slinky set came from eBay - yes they are metal $5 each.

4:1 balun on order to see if I can use my 15ft of 9913 coax to eliminate the Ladder-Line.

Dick   N3HKN
« Last Edit: March 08, 2011, 12:43:18 PM by N3HKN » Logged

Posts: 101

« Reply #1 on: March 08, 2011, 02:05:32 PM »

I always wonder about very sharp tuning of a tuner. It makes me think the tuner is having problems handling the load. I guess if the tuner isn't excessively "touchy" and the SWR isn't erratic I'd use it but keep an eye on it.


Posts: 18463

« Reply #2 on: March 08, 2011, 02:38:20 PM »

I think the Dentron used the old "Ultimate Transmatch" circuit, which has a higher
Q than the standard "T" network and is not quite as efficient.  There are often
multiple settings to match the same load, and with some of them the Q will be
high and a lot of the power can be dissipated in the tuner.

With a T tuner the most efficient setting is generally using maximum C and
minimum L.  I think the same is true of the "Ultimate Transmatch" circuit used in
the Dentron.  You might experiment and see if you can find other tuner settings
that work better.

The worst conditions for efficiency with many tuners are low impedance loads on
the lower bands (where the limited available capacitance forces them into higher Q
settings.)  I'd suggest trying an external 1 : 1 balun in place of the 4 : 1 inside the
tuner - if you have a low impedance at the end of the feedline, the 4 : 1 will step
it down even lower.  (You can rewire the internal balun if you want, but it may be
easier to try an external balun to see how much difference it makes.)

Posts: 1213

« Reply #3 on: March 09, 2011, 04:43:27 AM »

The sharp tuning effect is indeed due to a high Q effect of the antenna. The antenna is in such a configuration that the impedance of the antenna changes quickly off resonance. It is a common side-effect of loaded antenna, especially inductively loaded antennas like the "slinky" and trap antennas on their lowest frequencies.

This higher Q can often increase the voltages present on different parts of the antenna.


Posts: 243

« Reply #4 on: March 09, 2011, 05:02:40 AM »

So, it is quite likely that the Q factor is the basis for sharp tuning. No problem there. The high voltage may discourage the bats that came in last year. Smiley  I have a 4:1 balun on order that will go at the dipole/Slinky. If that allows me to run my 15ft of 9913 coax, without common mode radiation/current, than fine. However, if that fails I am looking at the MFJ auto-tuner that allows you to match very high SWRs if you keep the power below 150 watts.

Right now the ladder line is probably unbalanced due to proximity of coax that uses the same hole to get to the attic. Radiation is evident in the shack such that my PC speakers buzz with no power on them when the RF power goes above 40 watts.

Posts: 6147


« Reply #5 on: March 09, 2011, 07:24:33 AM »

Slinkys are crappy antennas.  As a single band antenna the best it's going to be is a linear loaded dipole made out of steel wire.  As a multiband antenna there's a good chance you will be operating above self resonance which will burn lots of power and also have phase reversals along it's length, which for a short antenna does nothing good for your pattern and efficiency.

Just make a doublet out of straight wire and call it done.  If you're going to use loading, do it in a way and a place that will contribute to and not degrade performance.

"Sharp" tuning tells me you're at an impedance extreme near the limits of the tuner.  Any number of combinations of antenna feed Z and feedline length can contribute to this and I think it has little to do with antenna Q/bandwidth.  It could also mean a resonance within the tuner which means your power is turned into heat inside the tuner.  Could be fixed by simply changing the length of the feedline plus or minus but I think it bears some investigation.

Mark K5LXP
Albuquerque, NM

Posts: 18463

« Reply #6 on: March 09, 2011, 09:04:42 AM »

Quote from: N3HKN
...I have a 4:1 balun on order that will go at the dipole/Slinky. If that allows me to run my 15ft of 9913 coax...

The ladder line will be better for multiband use.  I expect your tuner will be even more unhappy
feeding the antenna through coax, at least on some bands.

...Radiation is evident in the shack such that my PC speakers buzz with no power on them when the RF power goes above 40 watts.

That doesn't mean you have RF in the shack due to common mode currents.  More likely it
is because the antenna itself is so close to the shack that the RF picked up on the speaker
wires is enough to get into the amplifiers and make noises.  A more efficient antenna is
likely to INCREASE the interference simply because the RF level will be higher for the
same output power.  The solution is to shield or filter the speaker wires, not changing
the feedline.

Posts: 101

« Reply #7 on: March 09, 2011, 09:55:58 AM »

I agree with K5LXP. I have had bad luck with slinky antennas. I'd just try a dipole, IMHO!

Posts: 2080

« Reply #8 on: March 09, 2011, 01:01:43 PM »

The ONLY "pro" I've ever seen to Slinky dipoles (and I've played with them) is that, if you actually have the space, you can stretch or compress them as needed to make them resonant on various frequencies.  Kinda like a "helical" (and very manual) SteppIr dipole.

But, as other have noted, if you actually HAVE that space available, then you're much better off with a simple resonant dipole.  So the only "pro" of the Slinky dipole then becomes invalid.


Terry, WØFM
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