Odd radio behavior

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
MattFL
Posts: 26
Joined: Wed May 09, 2018 2:58 pm

Odd radio behavior

Postby MattFL » Mon May 13, 2019 11:23 am

This weekend I observed some odd radio behavior, and I'm just curious who else has noticed this.

Aboard a boat, one radio used a 3-to-4-foot-long antenna on top of the T-top; a second radio was a handheld attached to my PFD. Both radios were tuned to channel 68, a fishing tournament channel with lots of traffic.

Sometimes the boat's radio would receive a very loud and clear signal on 68, and my handheld would not receive anything.

Sometimes my handheld would receive a very loud and clear signal on 68, and the boat's radio would receive nothing.

This behavior was really odd and made me wonder how much radio traffic I was missing.

My handheld is a Standard Horizon HX870, and the boat radio was also Standard Horizon.

Antenna theory is fascinating, and I'm sure the explanation is complex.

Has anyone else experienced this type of thing?

Jefecinco
Posts: 892
Joined: Fri Oct 09, 2015 6:35 pm
Location: Gulf Shores, AL

Using Squelch to reduce radio receiver sensitivity

Postby Jefecinco » Tue May 14, 2019 9:29 am

I have noticed on our Standard Horizon VHF radio that I can eliminate much distant chatter on channel 16 by adjusting the squelch. We have a fairly high antenna and receive distant broadcasts much of which is commercial traffic around the State Docks in Mobile, AL. Perhaps a comparison of the squelch setting of the two radios could provide a clue.
Butch

jimh
Posts: 6114
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Odd radio behavior

Postby jimh » Tue May 14, 2019 9:56 am

The phenomenon described is radio wave propagation with fading. See

https://en.wikipedia.org/wiki/Fading

In radio communication the phenomenon of fading is often overcome by use of antenna diversity. See

https://en.wikipedia.org/wiki/Antenna_diversity

The situation described in the initial post is a good example of the benefit of antenna diversity in reception. By having two antennas (and two receivers) the chance of not receiving a signal ("missing something") was decreased. When one antenna-receiver-combination did not receive a signal, the other antenna-receiver-combination was able to receive the signal. This is the goal of antenna diversity.

jimh
Posts: 6114
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Using SQUELCH to reduce radio sensitivity

Postby jimh » Tue May 14, 2019 10:14 am

Jefecinco wrote:I have noticed [on a Standard-Horizon VHF Marine Band radio]...I can eliminate much distant chatter on channel 16 by adjusting the squelch.


On an FM voice receiver, the SQUELCH control can be used to increase the threshold level a signal must have to break the squelch, that is, to un-mute the audio output of the receiver.

In the case of modern VHF Marine Band radios (and often found on Standard-Horizon radios), the SQUELCH circuit is quite sophisticated and uses a very modern method (phase-locked loop) to assess if there is a coherent radio signal being received as compared to just random noise.

Increasing the squelch moves the threshold level necessary to un-squelch the receiver audio output to a higher signal level. This tends to mask output from weaker signals which have a low signal-to-noise ratio in the demodulated audio.

Jefecinco wrote:Perhaps a comparison of the squelch setting of the two radios could provide a clue [to the cause of the observed behavior of the two radios].


That a difference in SQUELCH settings between the two receivers mentioned in the initial post could account for the observed behavior is not really likely. If one radio had the squelch set for a higher threshold of signal level, when that radio broke squelch then the other radio with its squelch set to a lower threshold would also have broken squelch.

jimh
Posts: 6114
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Method to reduce receiver sensitivity

Postby jimh » Fri May 17, 2019 2:32 pm

Another method to reduce receiver sensitivity is use of an attenuator in the receiver's RF section. In the VHF Marine Band radio the attenuator IN-OUT switch is often marked with the terms LOCAL-DIST(ant) or LOC-DX.

Inserting an intentional loss between the antenna and the receiver will cause the effective receiver sensitivity to decrease more or less in proportion to the inserted loss. Note that the transmission line connecting the antenna to a receiver already has some loss, but in accordance with good design principles, the transmission line loss is usually kept to -1.0 dB or less. Inserting, for example, a -10 dB loss network at the receiver input will degrade the receiver sensitivity by 10 dB.

The practice of including the option to insert attenuation in the receiver input path on VHF Marine Band radios may have been a reaction to a government test of VHF Marine Band radios for susceptibility to blocking by strong local signals. In some ports the testing showed that many typical VHF Marine Band recreational grade radios were very susceptible to being blocked from receiving in-band signals from other boats by the presence of very strong out-of-band signals such as NOAA Weather radio broadcasts or other strong local radio transmissions. For details see

Evaluation of Marine VHF Radios:
Compliance to IEC Receiver Standards
NTIA REPORT 99-363

https://www.ntia.doc.gov/files/ntia/pub ... 99_363.pdf