Range symmetry between two radios


given two identical radios, why would one Radio1 receive Radio2 well but not the other way around?

The only difference is the antenna height and coax length. Here are the two setups:

Radio1 (Base Station)

  • Icom M506 (25W output)
  • 8-foot antenna (Shakespeare 5226-xt)
  • 50 feet coax (LMR-600)
  • antenna base is 70 feet above sea level.

Radio2 (Boat)

  • Icom M506 (25W output)
  • 8-foot antenna (Shakespeare 5226-xt)
  • 20 feet coax (RG-8/X)
  • antenna base is 6 feet above boat waterline (6 feet above sea level).

The Base Station can hear the boat very well at a distance of 10 miles. However, the boat CANNOT hear the base station at all. The voice is noisy and hard to understand. I’ve checked the output power (25W+), VSWR (<2.0), and sensitivity on both radios using Shakespeare ART-3 tester and they both checked out okay.

Am I mistaken to expect if Radio1 can hear Radio2, then Radio2 should be able to hear Radio1?
Is the total range not just the addition of each radios’ individual range?

Is the boat docked or under way? Is the boat at sea or on a lake? What is between the two stations, hills or trees? Or is there direct line of sight? What frequencies are you using?

If it is practical you might try putting radio #1 on the boat and #2 at the base just to see if you get the same results. In a perfect world you should be getting getting like results. Swapping radios might rule out any radio issues.

The boat is at sea. Weather was calm and there were no waves or swells. The issue occurs both while the boat is underway or dead in the water.

There is a building that could be partially obstructing line of sight.

I’ve not tried putting Radio1 on the boat and Radio2 at the Base Station. However, I’ve already replaced Radio1 with a different/old radio M506. I’ve replaced Radio2 with a new Icom M510 which has better sensitivity than the M506. Thus you’d expect better reception at the boat compared to the Base Station, but no. Same results: the Base station can hear the boat well, but the boat cannot hear the Base Station.

VHF Freq 156.250 MHz

The paths should be reciprocal - if the boat hears a very noisy signal, then there simply isn’t enough getting to the antenna connector - There are some interesting features. These antennas are two ⅝ wavelength dipoles in opposition, so they do not need a ground plane. Typical tall marine types that will be happy mounted on both metal and fibreglass structures - the tops of the fly bridge or on the horizontal tube with radars etc. The design has a fairly high take-off angle - so the majority of the emmission is from horizontal to 45 degrees or so upwards. With the height difference between the 70’asl base station, and the boat then you should expect less than perfect performance as not a huge amount of the signal goes down. This could easily account for the noisy reception on the boat. The snag is that this also applies to receive - so the boat should also be attenuated at the base station end, yet this appears to work fine. The difference in signal between noisy and filly quieting is not huge, but swapping the two radios would be a sensible thing to try. IF it is signal loss causing the receive audio to be hissy on the boat, then the boat’s transmission should be weak on the base station. I cannot imagine what is causing the difference, apart from a receive fault - or perhaps the base station transmitter throttling back. Ar you certain the VSWR is OK on the base station antenna? A poor match there could cause the PA to lower the output. Is you test meter working properly?

paulears, thank you that’s the word I should have used, “reciprocal”

Indeed, the output power of the Base Station or a receive fault at the boat is the really the only thing that would make sense to me.

I inserted Shakespeare ART-3 tester at the output of the Icom VHF radio, in-line between the radio and 50 feet of coax. Measured 23+ watts output power. VSWR showed good too.

I also checked the VSWR/Return Loss using an Anritsu Site Master Cable and Antenna Analyzer S332D in the following manner: I disconnected the radio and replaced it with the S332D (at the radio end of the 50 feet of Heliax LDF50-A coax). VSWR/Return Loss looked good across the relevant frequency range.

I suppose the spectrum/antenna analyzer and ART-3 meter could show a false “good” VSWR if there is a lot off loss in the coax. But that doesn’t make sense because the Base Station can hear the Boat well.

I could rerun the test but keep the ART-3 in-line and check if the PA lowers to say 1 watt.

I wonder if the Base Station and Boat are simply beyond line of sight. And the only reason I’m getting a bit of extra reception at the Base Station is due to the signal from the Boat bouncing off the water/atmosphere/building or bending past the horizon by happenstance.

That is the reason I was asking about being at sea. Math is not my strong suit but with the numbers you provided you should be well within line of sight. That being said, it doesn’t take into account the take off angles paulears mentioned. And that math is something I am not comfortable with.

There are all sorts of possibilities on the path, path loss and reflections, but 10dB boost or cut is applied to receive and transmit. If you were to calculate it, you’d have the initial power, the first cable loss, the antenna gain, the path loss, antenna gain and receiver sensitivity - there’s no mechanism for receive acting differently, apart from a receiver fault, or transmitter fault. I suppose the fault could be non linearity in the receiver - where sensitivity appears to drop more steeply as the signal strength at the receiver drops, but that seems doubtful. It would be possible though, for a noise source local to the boat to be giving the impression of low signal. If there is low level RF noise locally, this would mask the wanted signal, and it would get noisier? Something like and analyser or an RF explorer might show you local noise?