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SDR and Computer app to measure Antenna Signal Strength and SWR??

dave3825

Member
Premium Subscriber
Joined
Feb 17, 2003
Messages
2,429
Location
New York
#21
Newbee trying to improve indoor Police Scanner reception on P25 Phase 2 Simulcast weak signal situation.
Same boat here. Indoor only antenna at this time. FMPA (tcp out ) with DSDPlus does a good job for what I listen too. FMPA works better with weaker signals than FMP24.

If you know linux, try OP25. It does good on the simulcast site I listen to also.

I have a 1/4 ground plane antenna cut to proper calculated length and I also have used a small Tram multi frequency mag mount antenna mounted to a 16" metal pizza pan that works pretty good too but never quite satisfied with the reception.
Those antennas are omni directional. Meaning they receive from all directions. I made a moxon antenna tuned for 770mhz. That antenna is directional like a yagi sort of. https://forums.radioreference.com/threads/772-mhz-moxon-antenna-project.379514/

seems like the antenna with the weaker signal performs better on the simulcast tower I'm looking at
It could be that the weaker antenna (less db gain) is not pulling in much from other towers and is sort of attenuating the signal.
 
Last edited:
Joined
Sep 8, 2006
Messages
2,764
Location
Stockholm, Sweden
#22
If you're not using a simulcast capable reciever like the SDS100/200 then the best spot for the antenna are probably not where you get the strongest signal but instead where you shield off the signal from other towers the most except for one single one.

/Ubbe
 
Joined
Dec 25, 2008
Messages
3,095
Location
New Zealand
#23
Any radio that has a signal strength meter will have a circuit to drive the meter and to maintain the audio output more or less constant with huge variations in signal level - often called AGC (automatic gain control) or AVC (volume). The "S" meter you have noticed is marked in decibels (dB's - capital "B" 'cos it's named after Alexander Graham Bell) which is a logarithmic scale and make great for measuring the huge variations in signal level. Now humblego said "the antenna with the weaker signal preforms better" - or is it because you can see the signal change better as you move the antenna?

In fact the "S" meter is much more sensitive at low levels and gives a greater swing than at high levels - very non-linear. One "S" point at 1uV is about a 3 times increase, but one "S" point at 100uV is about a 30 times increase in signal - so a 3 times signal change at 1uV is easily spotted but a 30 times signal change at 100uV is barely visible.

Electronics are much better than not so long ago - a logarithmic amplifier on a chip is easily accomplished where a lot of tubes would have been necessary previously - hence colloquially the "S" meter was known as the "Guess" meter! My Ailtech Stoddart NM37/57A does a good job from 30 to 1000MHz and 0.1uV to 1000uV! Not cheap when new but about US$85 not working and surplus - a simple power supply fault soon had it going well.

DSCF1354 crop.jpg
 

UPMan

Uniden Representative
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Apr 19, 2004
Messages
13,263
Location
Arlington, TX
#24
FM and digital signals don't increase the volume with a stronger signal. AM signals do, but even those don't really work the way you describe.

There are two basic types of AGC referenced with scanners. Audio AGC tries to adjust the audio amplifier to provide relatively consistent volume output. RF AGC automatically adjusts the RF gain (or attenuation) to keep the incoming signal at an optimal level for the receiver circuit.
 
Joined
Dec 25, 2008
Messages
3,095
Location
New Zealand
#25
Weak FM and FMN signals still have volume fluctuations if they are below the limiting level in the IF amplifier - the AGC does not start to 'bite' until that limiting level is reached. Audio AGC is fine but on weak signals all you get is amplified noise along with the signal.
 
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