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19th August 2008 15:46

Alan Yates wrote...

Antonio,

Thanks for linking my site on your blog.

So you have a 16 metre long center-fed dipole? What frequency, 30 metres QRSS band? That's a little long for resonance around 10.14 MHz, it will look inductive so it will need some capacitive reactance to tune out the inductance, electrically shortening it. So much depends on the environment the dipole is in, so its actual impedance could be almost anything.

Because it is fairly close I'd recommend a Z-match style circuit at the feedpoint (but that might not be easy to install and maintain, you might like instead to run open-wire line down to a matching unit in the shack). The bonus is it can reflect an admittance of either polarity to the antenna and "tune" it, while looking like a flat load to the transmission line once tuned up. I've been meaning to write up the matching unit I built for the loaded dipole, it is quite effective and would probably work in this situation.

In short, it is a T120-6 core with 14 turns (I think, can't remember, I calculated the turns based on the inductance required to resonate mid-way on the polyvaricon). The antenna is connected across another 6 turns on the same core. The 14 turns is resonated by a 6-210 pF polyvaricon and another polyvaricon feeds the 14 turn winding at a tap (about 8 turns from the cold end) from the coaxial feedline. I'm a bit shaky on the details, I'll pull it down and photograph it in another article.

The great thing about QRP is that you can get away with using cheap and small components, like polyvaricons and toroidal cores. Building a QRO matching unit is expensive just in the Copper required.

Regards,

Alan

19th August 2008 05:43

ik1hgi Antonio wrote...

Hello Alan very interesting your site I put a link on my blogspot 500mW amplifier in order to visit other OM, if you ask me can understand! on a long dipole 8 meters per arm, I want to match the 4 qrp, with power from 100mW on an antenna? how can I achieve?

73 Antonio ik1hgi ( GRAZIE )

4th August 2008 10:59

Alan Yates wrote...

Ron,

Thanks again for the signal reports, I've added it to the beacon report summary table.

I am gearing up for switched antenna experiments where the beacon will automatically switch between two different antennas allowing real-time comparisons between the antenna performances to be made. Would love to have future reports when this comes online.

Regards,

Alan

4th August 2008 01:05

Ron wrote...

Hello Alan, This is my second report to you. I am listening to your beacon again - 1447z (0018hrs local) Monday 4th August 2008. RST 479.

Very stable signal. Still sounds very good. QSB. My equipment - FT817ND, T1 Tuner and 35 meters of wire. QTH Tanunda - 70km NE Adelaide. I have listened several (many) times since my first report. Signals have been variable between strength 2 and strength 7. Good work for 1.5W and base loaded whip Alan.73. Ron VK5AKR

19th July 2008 22:42

Alan Yates wrote...

Peter,

Thanks mate, good to hear.

I think I'll run some coax to the shack from this antenna. It is quite narrow band of course, but it clearly gets out better on 80 than anything I've used in the past (except the end-fed half-wave but I can't leave that up permanently).

Regards,

Alan

19th July 2008 15:22

marxy wrote...

Alan,

beacon sounds very good today (3pm). S6 at the moment.

18th July 2008 10:43

Alan Yates wrote...

Juanjo,

There are 70 turns of 1.8 mm wire. The former is 40 mm OD, the winding length is a little less than 130 mm. The total inductance is about 62 uH.

This agrees pretty well with the coil calculator if you put the average diameter of the current sheet into it, at around 42 mm. I recently had a question emailed to me about the coil calculator and used this as an example of its relative accuracy.

Regards,

Alan

18th July 2008 01:21

Juanjo wrote...

Hey Alan, how many turns does the loading coil have? I have tried to find it out in your article to not avail. Thanks for your answer in advance.

17th July 2008 13:40

Alan Yates wrote...

Ron,

Thanks for the report. At around 1200 km this is the farthest I've got so far, and finally gives me a non-local one to the west.

I was beginning to wonder if my antenna pattern was favouring northwards propagation. I should build a field strength meter and take a hike around the compass at a few km distant measuring the pattern. The antenna is *far* from optimal, the actual radiated power is probably only a few percent of the TX power.

Regards,

Alan

17th July 2008 03:58

Ron VK5AKR wrote...

Hello Alan, I am listening to your beacon - 1747z (0318hrs local) Thursday 17th July 2008.

RST 439. Very stable signal. Sounds very good indeed. Just while listening now the beacon is increasing in strength - QSB. My equipment - FT817ND, T1 Tuner and 35 meters of wire. QTH Tanunda - 70km NE Adelaide. I am going to look into your site more when my visitors go on Saturday. 73. Ron VK5AKR