31 January 2010

Multiturn loop matching

I couldn't resist to see the LW loop hanging in the shack, unused. The quickest change I could apply was to add a transformer to match the loop high Z (circa 1800 ohm, simulated) to 50 ohm.

The transformer requires approximately a 6:1 turns ratio, for a 36:1 impedance step down.

From the bag of unknown toroidal cores I picked a small one, black, the size of most familiar T37-x. It was used as an RF choke in some computer equipment. With a thin wire I wound 24 turns on the primary (loop side), so that I would need 4 turns on the secondary (RX side). The DVM LC meter measured 900uH from those 24 turns: high, as expected! The material should be appropriate for the intended frequency range!

The result on the receiver is astonishing. The previously observed lower Q on the high end of the tuning range is fixed: the tuning is razor sharp on the whole range. The new range is 150 to 570 kHz. The transformer does not seem to be lossy in LW/MW as observed signals are equal if better than before.

I still have not understood where the figure-8 radiation pattern lies: orthogonal or on the loop plane? Sources say othogonal, but I observed it on the loop plane with aeronautical NDBs. I mean, if on the loop plane was built a yagi, I orient it for the best signal. Anyone?

28 January 2010

DX-pedition movies

Through an Italian HAM forum I got to see two HAM related movies about DX-peditions.

The first hosted on blip.tv is about Russian Arctic Islands in 2001. It is one hour long.

The second is from 1976 on Google Video, 7J1RL Okino Tori-shima, full of music background from 70's.

Both are very instructive for anyone considering a field activity of any sort, even the apparently simple task of operating from the nearby park.

Use the search engine on both websites to locate these videos.

One night of WSPR on 500 kHz

One night of WSPR on 500 kHz brought in only PA0A, that had stopped transmissions shortly after midnight UTC.

I will resume listening only once I have done some changes to my setup, like installing the antenna outside or building the loop preamplifier.

25 January 2010

Longwave/mediumwave square loop - picture

I think it's time to show my homebrew multiturn LW/MW square loop. Here it is, hanging from the library on the back wall of my shack. In this position I have received PA0A on 500 kHz WSPR.

CFL light bulbs seem to have no ill effect on the background noise, except for some birdies that can be heard occasionally. Also SMPS in the shack don't make a difference.

The loop still has no impedance matching transformer, so the high Z goes directly into the FT-817 antenna socket. That's probably the next improvement planned.

22 January 2010

First WSPR reception on 500 kHz

Just for the fun of it, since the eeePC was running, I tuned in the 600m band (500 kHz) and let WSPR run while we had dinner. Receiver: FT817ND.

As antenna I used my multi-turn 1m square loop inside the shack, turned so that I minimized the received noise. Moreover the loop tunes about 230 to 450 kHz, with a very low Q at the high end, so 500 kHz is a bit border line for its current configuration.

Nevertheless, buried in the noise I could receive PA0A about 900km away. Go figure on the NDB band I am able to read only 3 signals, the furthest being just 60km from my antenna!

So the loop is not that bad. The location needs improvement. I will move the loop outside, on the balcony. Then with a warmer season I might try to move in a less RF-polluted environment.

21 January 2010

Longwave/mediumwave square loop

Pushed by the achievements of authorized HAMs using the 500kHz/600m band, I decided to rebuild my multi-turn square loop for long/medium wave.

I played with it maybe 10 years ago but I still had all parts laying around. It is composed of:
  • 4 meters of computer flat cable, 26 wires (probably), soldered "shifted-by-1"
  • two wood sticks 1 meter each
  • one variable capacitor (whatever you have at hand, even a trimcap will do)
Initially I will hang the loop on the shack window, hoping not to catch too much noise from the building, starting from the shack CFL lamp.

The loop will be 25 turns in parallel with the capacitor, coupled with one full turn loop to the receiver. Probably this configuration has a high impedance, about 2kohm (how to measure it? my MFJ259 doesn't go that low), so I might also try a step-down transformer with a surplus toroidal core salvaged from somewhere.

If the loop will not pick too much noise I will try to add an active amplifier in between the antenna and the receiver.

18 January 2010

Prematurely failing batteries (AG13, LR44, ...)

Most toys at our home use either AA/AAA batteries or 3xAG13/LR44 button cells.
I had once bought them online and I was relatively satisfied with their quality. Then I had to re-supply at the local store and paid the incredible amount of 1.5€/cell. They had even more expensive ones!

Out of 4 batteries, one was dead. Other 3 lasted much less than those that came with the toy.

During a desperate need of one of those toys last night, I got hold of my DVM and measured voltages across those "exhausted" cells.

40% of them that had been marked as "unsuitable" for toys were at 0.5V or below. Way below: they showed polarity reversal! The remaining cells were measured to be 1.2 to 1.4V and, not surprisingly, when mixed with good cells, brought a couple of toys back to life.

You may have to do some interactions until you find a working trio, but don't throw them to recycle bin right away because the fault might be in just one cell, not in all of them!

Apparently they should have capacity in excess of 150mAh, I think that a simple testing clamp can be arranged with a resistor mounted on a clothes peg...

04 January 2010

Calendario IAC VHF/UHF/SHF per agende elettoniche

Grazie a I1BPU che ne ha assunto la gestione per il 2010, l'Italian Activity Contest VHF&up continuerà ad esistere anche nell'anno 2010.

Ho pubblicato il calendario IAC 2010 su un file separato per non creare problemi a chi avesse già utilizzato il calendario dei contest da "weekend".

Lo trovate sul mio sito: www.paolocravero.tk

Il mio calendario dei contest si può consultare online su google o sottoscriverlo (iCalendar).