Hi all,
I am using a homebrew radio and did some changes recently. This weekend I have been activating couple of summits in OK and I had a feeling that all signals were very weak. All stations calling me on 40m had signals 51-55, not more. So is it the conditions or do I have some flaw in my radio?
Marek, OK1BIL
In reply to OK1BIL:
Hi Marek,
simplified test whether the radio sensitivity is sufficient, can be realized in several ways, especially:
noise level should drop audibly:
when disconnecting the antenna (pream on, att off), however, this does not work well if the problem is with the antenna
when the preamplifier is switched off
when the attenuator is switched on
73
Karel OK2BWB
In reply to OK2BWB:
Sure, I have made a couple of tests, but I do not have equipment to really measure sensitivity of a receiver at home. What more, I do not know what was the sensitivity before the mod.
Making an audio test is exceptionaly hard with this radio. It is very simple NE612 design lacking any of the advanced features like preamp, user tunable attenuator or AGC.
My subjective feeling is that what used to be 59++ is now 58 and what used to be 59 is 55 now. So I am just looking for someone to confirm that conds were good/bad this weekend on 40m before I blame the radio.
Marek
In reply to OK1BIL:
Marek
You may find the bands were noisy due to the CME (coronal mass ejection) which hit sunday…
So you before the radio takes the blame, check out http://www.solarham.net/
It is very pronounced if you listen to 18.7Mhz !!
Also check this site HF Propagation and Solar-Terrestrial Data Website
73 Adrian
In reply to OK1BIL:
Good approach that should work for most simple but well-designed radios is to short-circuit the input of the first NE612 LNA part (NE602, or any other input amplifier) via a capacitor (10n or so, not to violate input bias settings). The level of noise in the headphones should audibly drop.
This is a fairly universal test that allows to very quickly assess the sensitivity of the radio.
Karel
In reply to OK2BWB:
Thank you all for suggestions. I remembered today at work that I tested the radio after doing the mod and everything seemed to be okay. So I took an ohmeter and checked coax and balun and guess what, the live wire in the balun did not conduct. So I was basically transmiting into halfwave inverted L ground wire. Interesting! I have activated at least 4 summits and the final transistor didn’t go to silicon heaven!
I am now thinking how to avoid such situation in future. Do you have any tip for a very very very small QRP SWR meter? Something with LEDs maybe?
In reply to OK1BIL:
Have a look at PicSWR by G3VPX, this may be suitable for you. It works with LED’s as a display and talks to the bar graph via rs232 as a display for ref / fwd and SWR measurements, good for an inbuilt meter.
These small QRP projects can be great for SOTA, they draw even less power then the FT817, and they take a bit of skill to use.
Just a note with gain, my Minima is very quiet with rx noise, but it is surprising how little RF gain you need to work weak stations buried in the noise. Don’t be tempted to build a Pre-amp unless you are finding on higher bands the sensitivity drops, in which case I would look at the mixer circuit first, the 602 isn’t the best out there.
Have fun with it !
Jonathan
In reply to OK1BIL:
I am now thinking how to avoid such situation in future. Do you have
any tip for a very very very small QRP SWR meter? Something with LEDs
maybe?
Yes. Use a Tayloe Bridge. It’s all I ever use.
Dead simple to make and you always have a resistive load during tune up.
Here’s a kit if you can’t do it yourself (other products are available)
http://www.qrpkits.com/swrindicator.html
72 Pete