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14 mai 2016

APRS pocket transceiver bazat pe Arduino

M-am apucat sa experimentez cu un Arduino niscaiva traznai de APRS. Pana la urma a iesit un tracker care poate sa so receptioneze traficul APRS, inclusiv mesajele.
Transceiverul este un DORJI, v

ia LCCOM, placa uC este custom dar merge bine mersi si un Arduino UNO cu oarece componente pe langa ea. Cateva rezistente, condensator


i si un potentiometru. Comenzile la DORJI nu necesita curenti mari astfel ca un port I/O Arduino suporta fara probleme PTT ul.

30 aprilie 2016

Antena dipol compact pentru banda de 60 m


Dimensiunile dipolului scurtat
Recenta permisiune de a utiliza un canal intre 5363,5 – 5366,5 kHz a prins pe nepregatite radioamatorii YO.

Am auzit in diverse discutii ca se intentioneaza evaluarea propagarii cu antene verticale, complet improprii efectuarii de legaturi la distante de 100-500 km in conditiile in care permisiunea de utilizare este pentru evaluarea posibilitatii de utilizare in situatii de urgenta pentru sprijinirea activitatilor de cautare-salvare.

Drept pentru care, solutia optima este utilizarea unui dipol la o inaltime de circa 0.20 - 0.25 din lungimea de unda, adica la circa 5-6 m inaltime. Aceasta ar fi inaltimea optima pentru a reduce pierderile in sol, dar este acceptabila pentru a beneficia de efectul NVIS si o inaltime de circa 3 m.

Pentru ca si eu, ca majoritatea radioamatorilor am probleme cu spatiul, un dipol de 2x15m este inabordabil (intentionez sa il amplasez in podul casei de la tara) , am realizat si testat un dipol scurtat, cu anvergura de doar 12m.

Sigur ca un astfel de dipol va performa mult mai prost ca un dipol full-size, cu circa 5 db, dar apropierea de sol reduce, de asemenea, si zgomotul captat pe unghiuri mici de radiatie, ceea ce nu este tocmai rau, avand in vedere ca testele se efectueaza pe teritoriul YO si nu la DX.


Solutia gasita pentru scurtarea lungimii electrice este introducerea pe fiecare brat al dipolului a unei bobine.





Realizarea practica este extrem de simpla si necesita o teava de instalatii, PVC, cu diametrul de 40mm, pe care vom bobina spira langa spira un numar de 42 de spire cu sarma de instalatie electrica izolata cu PVC, cu grosimea de 1.5 mm.


Aceasta bobina se va amplasa exact la jumatatea fiecarui brat al dipolului, conectandu-se la acestea cu borne tip papuc, sertizate pe conductorul radiant.


Incepem prin a taia doua bucati de fir de Cu multifilar de 1.5mm izolat cu PVC, de 3 m lungime. Acestea sunt cele care se conecteaza la izolatorul central.
Eu am folosit si un UnUn 1:1 (choke-balun)/



Taiem alte doua bucati de Cu multifilar tot de 1.5 mm dar in lungime de 3.5 m. Acestea vor fi legate la extremitatile dipolului, de la bobina la izolatorii de capat.


Pregatim doua bucati de teava PVC de circa 20 cm pe care vom bobina, spira langa spira, cele 42 de spire de Cu 1.5mm izolat cu PVC.


De aici incolo, fiecare isi va folosi imaginatia pentru a realiza conexiunile. Eu am folosit suruburi de 4mm cu piulite tip stea din inox. Peste bobine am aplicat un strat de banda izolatoare pentru imobilizarea bobinei.
Tevile au gauri pentru fixarea mecanica a firelor care constituie bratele dipolului iar ca element central am folosit un suport pentru dipol cu descarcator incorporat.


Evident, la capete se vor folosi izolatori ceramici sau de plastic cu striatii pentru a impiedica formarea arcului electric in conditii de umiditate.


Cateva date despre bobina:

Frecventa proprie de rezonanta:  circa 21 MHz
Inductanta: 35 uH (necesar teoretic: 32 uH)
Rezistenta @5 MHz: circa 1350 Ohm.
Factor de calitate Q: circa 400.

IMPORTANT:

La capetele dipolului, lasati o bucla de circa 30 cm pentru fiecare brat; veti taia cate 1 cm pentru a obtine acordul antenei dupa amplasarea in pozitie. Masurata aproape de sol, frecventa de rezonanta va creste odata cu ridicarea in pozitia finala!

Cu dimensiunile prezentate, nu a fost nevoie sa tai din fire.


27 aprilie 2016

APRS at smallest footprint

Today I received the APRS modem boards.
They are made around a ATMEGA328 and can be programmed with Arduino IDE.
All they need is a small GPS and a transceiver. Of course, the exigent hams can put a small I2C LCD to have a nice visual feedback.
After the Easter Holiday I will put them to work and I'll keep you in touch!

73 de Adrian



26 aprilie 2016

RFSM2400 and WinLink Winmor

Today I did some tests focused on the file transfer speed using two soundcard modems.

RFSM-2400 receiving test
RFSM-2400 is a freeware client program which can transfer files  over a radio channel, using MIL-STD-188-110A standard (300Hz to 3.3 kHz-2400 baud).

Winmor TNC, is a donationware program with a modulation scheme that resembles very close with the one used on PACTOR modems. The speed is bandwith related and is adaptive between 200 Hz and 2000 Hz.

RFSM 2400 can't be used to send files to internet servers but the Winmor is widely used with RMS Express to connect to various iGates around the world to send e-mails.

For tests I used a FT-991 connected

Winmor TNC receiving test

to RMS Trimode and to RFSM-8000 (can act as a server) and a IC-7300 connected to the RFSM-2400 and RMS Express with Winmor TNC.

I sent a jpeg file in size of 56.0 KB (57,344 bytes or 448.000 bits) with RFSM-2400 and with RMS Express via Winmor TNC.

The radios were set on 51 MHz, on dummy loads at 5W with a distance of about 1 m between them.  Both were on Wide and the signals on S-meter were peaking S3-S5. The modulation was set to not activate the ALC.

The results were astonishing! Via RMS Express, I sent the file in about 20 minutes.
Using RFSM-2400 the same file took less than 4 minutes...

So, the quick and dirty test have some conlusions:

1. When e-mail is important, Winlink + Winmor TNC is the one. There are many iGates on HF and is impossible to miss one, especially if you use the VOACAP tables to estimate the best channel for the given day and hour.

RFSM-2400 receiving private file (mail)
2. Do not try to send big attachements via Winlink! Instead, focus on smaller files, plain text or, if attachements are mandatory, use a good compression software.

3. If ad-hoc networks are the scope, RFSM is the best! You can define a FTP directory and  mailboxes  for the radio stations and each station can upload emails download their own. The FTP is very usefull for sharing files from one station to another. Good for a regional or national Emergency network.
A plus for RFSM-2400 is the chat and beacon mode!


15 aprilie 2016

Playing with the front end of the IC-7300

I heard some rumors about how bad is the IC-7300 front-end and how unsuitable is for hunting low signals so I want to try to push the input to the limits.

So, I put a power splitter to the antenna input of the radio and feed it with two signal generators.
One set on the receiving frequency at -100dbm and one at various frequencies, set at 0dbm. You see, the difference between the signals is huge in terms of input signals...
I am not trying to say that this setup will produce the same result as in real life but is a base for some evaluation of the receiver.

Therefore, the receiving signal is at S1 (-100 dbm) and the perturbation will be at about S9+27 db, according to the IARU reccomandation for HF S-meters.

The radio was set to work with no preamp nor ATT, and the NR OFF.

At S1, the signal was full readable with some noise. With NR ON set at 5, the noise dissapeared and the receiving signal was full quiting the speaker!


When I was applying the perturbation outside the selected filter, OVF quick came on the LCD and the noise increased but the signal was still readable. The NR made the signal nice and clean, altough not so clean as the first time.



When playing with the perturbation signal inside the same filter's passband, the noise increased a little bit but not as I expected.



When the OVF was LIT, there was some artifacts on the waterfall.

The OVF and the artifacts was gone when I reduced the RF gain (instead of 12, the RF gain was set to 10).
The signal was still readable but with a price in noise level... The NR does it's job very well in reducing that noise.

By reducing the perturbation signal at -6 dbm, the OVF goes down and so did the WF artifacts.
At -12dbm, only a slight increase in the noise was there, nothing to upset me, even without the NR.

This beahviour was observed anywhere in the HF bands. I did not test yet the VHF (50+70 MHz).

Indeed, the out of band strong signals are able to put some noise in the receiving band but is not so bad as stated on some other places on the net.

However, the biggest problem is with the harmonics; I set the signal generator at 7.150 kHz at 0 dbm and I found a S9 signal on 14.300 kHz and a S8 on 21.450 kHz! This could be a problem when working on a crowded area (crowd of ham, of course).

A theoretical approach say that the best antenna for this radio will be a monobander or a non-resonant antenna tuned with a AH-4.

As a personal experience, I used the radio with a multiband vertical and find nothing to complain as the nearest ham is at about 8 km from me and there is a hill between us.

The test was long and complicated as I measured also the S/N ratio but nothing spectacular shows... Only the conclusions above.

For this test I used a HP 8656B and a IFR2945A.

73 de Adrian YO3HJV

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