There is a physics phenomenon called multipath…and I believe this has a lot to do with why elevated layouts work well on RC…and ground based ones work less well.
We all have experienced multipath…while listening to AM radio stations in our car. As we drive away from a distant AM station…the radio signal gets better…then worse….then better…than worse. This is because the AM radio signal that we receive in the car is getting to the car by multiple paths….on might be a direct path, radio station antenna to us….or one might be on bounced path, radio signal bouncing off the ionosphere then to us. Both signals are at the same frequency…because the radio station only transmits one…but, radio signals are waves…like you see on the ocean. When a radio signal goes further (or shorter actually) in its trip to your radio in the car, the direct wave and the bounced wave TOPS shift in placement in space because they travel different distances.
Now…radio receivers ADD all the signals they receive. If the direct wave and the bounced wave get displaced by exactly a half wave length, one cancels the other because when the radio adds the amplitude of two waves of the same height that are displaced by a half wave length....the positive part of the direct wave is added to the negative part of the bounced wave…and you get zero…no signal. Now, in real life, the direct signal and the bounced signal don’t have the same height…so, you don’t completely LOOSE the signal…but you do hear the volume on the radio go up and down…or static increases…or you lose the station for a while and then it comes back. And..the direct wave signal is NOT always bigger than the bounced wave…the point is that the two signal paths can lead to signal cancelation. The place where that signal cancelation exists is a called a multipath null.
Now…how does this multipath stuff effect RC? Well, the water in the soil in a yard is like the ionosphere…the water reflects radio waves. So, when you are using an RC system, the signal the engine is receiving comes from a direct path from the transmitter to the engine…AND, a bounced path from the soil to the engine. Now, at 27MHz, the wavelength is like 6’ long…so the half wave length is like 3’…the length of the antenna wire. So…if the signal bounced and the direct signal get shifted 3’, the signals cancel. This is what is happening when engines run on the ground…and is why walking with the engine seems to make control better. If the moisture level in the soil is 18” down…and you’re walking with then engine…then the bounced signal goes 18” down…and then back up…3’…and you get signal cancellation at the engine. That’s simplistic…but it makes the point.
If the moisture level is 3” down….then there is a place where the angle of the radio wave from the transmitter to the engine will go through the soil for 18”…be reflected…and then go out of the soil for 18” to the engine…and you got cancelation again. This is why on ground based layouts engines go out of control for a while when you stand off from the engine. The engine might have to move 10’ to shift those direct and bounced signals out of cancelling one another…and to you, it means you have no control during that period. If you’re standing next to the engine…it might only have to move a foot before the signals stop cancelling each other.
It also explains WHY walking with an engine on the ground, the control is better. In reality, the signal that bounces on the ground moisture is “attenuated”…meaning reduced in strength. The direct signal is NOT. Therefore, when the two signals add at the receiver…the direct signal is strong enough to overcome the strength of the bounced signal…and the radio can see the signal…and life is good. Things work as advertised.
OK…so why does an elevated layout work better? Even on an elevated layout, the engine receives both the direct path and the bounced signal. Well…the answer is…we hold our transmitters at about the same height as the track…even when the track is way out there. When the train is 100’ away, the angle from our antenna to the train is NOT real different that when it’s 3’ away. That means…the direct path is always getting to the engine strongly and the bounced path is going through a LOT of soil…even if the moisture level is only down a few inches. If the angle the bounced signal hits the soil is a few degrees, it goes through feet and feet of dirt on the way into the moisture level…then bounces (is reflected)….and feet and feet on the way out of the soil That reduces the strength of the bounced signal greatly…and the direct signal dominates…and you maintain control.
To be complete, yes….there ARE multipath nulls even on an elevated layout…but it turns out that multipath nulls tend to be more concentrated at low elevations. This is why missiles that attack ships are designed to fly just a few feet off the ocean surface….and why fighters and bombers fly very low on strike missions…they’re trying to fly IN the multipath nulls at low altitude of the radars they are trying to get past…or as has the pressed states it….fly under the radar.
That’s the basics…but have ya ever wondered why folks are switching to Airwire and away from TE, RCS, etc? Cause it “works” better. The answer is in the frequency. Airwire is a 900MHz system. TE/RCS/etc are either 27MHz or 75MHz systems. Well….what makes that important? Simple…wave length. The wavelength on an Airwire system is several inches long. The wavelength on the TE/RCS/etc is several feet long.
So, why does that matter? Well it means that the time when a moving engine has its direct path signal and it’s bounced path signal exact, say 2” apart (a half wavelength when the signals are canceling)…is VERY short. The engine might only have to move a ½” to get back a signal that the receiver can see. So…you don’t see the effect of multipath. Now think about the 6’ wavelength situation…the TE/RCS/etc frequencies…it means the engine has to move several FEET…maybe yards…before the signal is no longer canceled. These are the time periods that we all bitch about.
There are other things that impact this too…the most important one being how you deploy the antenna. LSers make two mistakes…all the time. First, they wrap the antenna around some cardboard and they put it in the roof of the tender. Secondly…the point the transmitter AT the engine.
Let’s talk about number one…wrapping the antenna wire. An antenna that is properly “tuned” should collect at least a half wave length of energy from a transmitter….just plain physics. So…if you want to properly “tune” you antenna…you want it stretch out completely…and they are cut to the length of a half wavelength by the radio maker. When you wrap the antenna around something…you’re trashing the antenna’s performance.
Second point…aiming the transmitter. Well…it’s NOT just the transmitter…it’s also the receiver antenna. There are two things going on. It turns out (again physics) that the side of an RC antenna transmits/receives the strongest signal….and the ends transmit the weakest signal. This is called the antenna pattern. So…if you “point” the antenna at an engine….like a gun…you are substantially reducing the level of the signal that the engine receives on direct path…AND…guess what…you sending MORE signal though the ground bounce…and it’s reduced in strength by the soil. So pointing is NOT good.
Last point…why do RC cars work good at 27 and 75 Mhz….and not LS engines. Answer….it’s the old antenna pattern again…and something called polarization (which I’ll stay out of). RC cars have VERTICAL antennas….that are a half wave length long. So…if you hold your transmitter with the antenna UP..and the RC car antenna is UP…you’ve got the best signal path possible….antenna side to antenna side. If LS trains could have vertical antennas…RC control would be a lot better….
BUT WAIT….we CAN have vertical antenna in a LS train!!!! How? Use a higher frequency radio….like an Airwire where the half wave length is like 2”….meaning….you can stand the antenna UP inside the tender because its only 2” long. This is a case where bigger is NOT better.
And that ends my explanation of multipath effects on LS engines….
Be advised that there are a LOT of other aspects to this….like FM vs AM…digital vs analog…polarization…laws/regulations that get into the LS RC discussion. I don’t feel they are as significant as the basic physics issues of multipath and frequency …but others will argue otherwise. What’s above is just my personal opinion…having been a radio/radar electronics engineer 30 years ago.
Hope this isn’t as clear as mud….