Railfan Technology > Transmissions

What is the main reason that the scanner can't pick up both sides of the conversation between the DS and to that of the road crew despite having a antenna tuned to the frequency spectrum ? This problem seems to be more prevalent with my Uniden handheld over that of the mobile (same brand) I use in my truck. But there are times where when the mobile also experiences the same loss of conversation as with the handheld. And I would think that being in what I would consider a good clear area free from any radio interferences would give me a better overall listening show but somehow it doesn't. Sure, I know that the transmitter for the DS atop the hilltop is more powerful than the engine radio, but does not the same radio signal exist between the two radios or does it come down to the radio/scanner sensitivity. Would owning a Motorola without the transmission crystal in it do a better job ?

I ask this because there are times or days when the both sides of a conversation can be heard and there are some days where the distance thing from which the area being listened to is fairly far away while on other days, just the opposite is true. Sure, it can be because of the atmospheric conditions, but wouldn't that be more consistent rather than sporadic ? I've heard a lot when nearby the main yard with a lot of crews or a crew talking with the DS, but at other times while up in the mountains and despite being at the same if not the exact level of the ROW, the conversation seems to one-sided AKA only hearing the DS talking.

If there is only one or two hillside towers/repeaters in the area or the surrounding area, it the signal between the DS and the crew more "line of sight" due to FM "straight line signal" frequency and any radio or scanner out of this "line of sight" is only going to hear the dominant side ?

What gives ??

The DS radio broadcasts a stronger signal from an elevated location. The locomotive radio is of lower wattage and may have the signal blocked from your line of sight by a cut or hillside. Some channels use two frequencies with the DS transmitting on one of the channels and the train crews broadcasting on a second frequency. If you do not have both frequencies programmed in you only get one side of the conversation.

VHF radio wave propogation is also heavily influenced by weather conditions, which is why "certain times of day" might seem to play a factor in when you may or may not be hearing both transmissions. Temperature inversions, that is layers of warmer and cooler air in the lower atmosphere can cause signals to travel many(and sometimes hundreds of)miles farther than line of sight. Frequently winds will subside during overnight hours, allowing air to cool and layer which contributes to the process(which is why in the early morning you will often note very long receive distance). If you consult a weather map, you will occasionally see High Pressure systems with very tight millibar patterns surrounding them, as the backside passes your location you should note longer than normal receive distances(and especially if the system stagnates or moves very slowly away from you-which is when things can get really crazy).

And yes, a high quality tranceiver like a Motorola(and many reasonably priced amateur transceivers available these days)with a proper antenna, will out perform most any scanner currently in production.

A good antenna is a certain plus, a good antenna that is mounted higher than the surrounding rooftops and obstructions is even better, watch out for that lightning thing though...

It sort of boils down to the signal source and the path between that signal source and your antenna. The dispatchers radio (antenna) is usually at an elevated location. Maybe it's mounted 20 feet above the hump tower and that hump tower might be 60 to 100 feet above the ground. Additionally, the dispatcher's radio is usually higher power, say 60 to 100 watts. BTW, the higher location of the antenna is an advantage for the dispatcher when listening to trains out on the road. Another advantage the dispatcher has is the ability to use an antenna that has gain. Again, that works both ways also.

On the other hand, the radio on the train might be 25 to 40 watts and the antenna is only 15 feet above the ground. The antenna is not a gain antenna. Depending on your location, you receiver, you antenna, the elevation of your antenna, your feedline (if any), the terrain where the train is, where you are, the objects between you and the signal source, and the phase of the moon (not really,) you could probably hear the dispatcher 20 miles away but not hear the train 10 miles away.

Toss in another situation where someone is walking along using one of the handi-talkies... 5 watts, 5 feet above the ground, and an antenna that performs with the equivalent of loss.

As they say in real estate, "Location, location, location." I'm a ham and one of the repeaters I've used is located 700 feet above the ground on a hill puts it about 1100 feet above average terrain. I can talk to it with 1 or 2 watts from 50 miles away with a handi-talkie inside my car. If I try to talk to another ham from my car to his car and we both have 15 to 25 watt radios, we might have problems communicating.

You compared the hand held scanner to the mobile scanner. I suspect the hand held has the rubber duck antenna and the mobile scanner has at least a 1/4 wave (17.5 inches). Just that slight advantage the mobile has will present an improvement in performance that you should notice, and I think you said that it does perform better.

One of the places I like to go for photos is in a valley. There's a really nice trestle there. I can all ways hear the dispatcher but I can't hear train until they are about 4 or 5 miles away. Once I get out of that valley, the reception improves greatly.

I've got a few notes on receivers and antennas at http://members.trainorders.com/wa4umr/ It's a work in progress but you might find some information there helpful.

Bottom line, this has been sort of a "beat around the bush" answer. I haven't really told you for sure what the problem is, just a few ideas of what I think it might be.. a bunch of guesses. I hope you find something useful here. Drop me a line if you have further questions and I'll try to answer them. Also, Tim, K3HX may chime in here. He is a great resource in matters of this nature.

John

Pretty much agree with the points made by the posters although I've

not found weather or time of day to be a significant factor in routine,

day-to-day reception of VHF signals.

Be Well,

Tim Colbert K3HX

As pointed out, weather can be a factor, especially temperature inversions. Living north of Chicago in Lake County, I often picked up railroad transmissions from northern Indiana when inversions were taking place along Lake Michigan in spring and summer. On one occasion, I heard a hotbox detector in lower Michigan. Normally, that would not carry beyond 3-5 miles.


Pepperidge

Wizard Wrote:
-------------------------------------------------------
> What is the main reason that the scanner can't
> pick up both sides of the conversation between the
> DS and to that of the road crew despite having a
> antenna tuned to the frequency spectrum ? This
> problem seems to be more prevalent with my Uniden
> handheld over that of the mobile (same brand) I
> use in my truck. But there are times where when
> the mobile also experiences the same loss of
> conversation as with the handheld. And I would
> think that being in what I would consider a good
> clear area free from any radio interferences would
> give me a better overall listening show but
> somehow it doesn't. Sure, I know that the
> transmitter for the DS atop the hilltop is more
> powerful than the engine radio, but does not the
> same radio signal exist between the two radios or
> does it come down to the radio/scanner
> sensitivity. Would owning a Motorola without the
> transmission crystal in it do a better job ?
>
> I ask this because there are times or days when
> the both sides of a conversation can be heard and
> there are some days where the distance thing from
> which the area being listened to is fairly far
> away while on other days, just the opposite is
> true. Sure, it can be because of the atmospheric
> conditions, but wouldn't that be more consistent
> rather than sporadic ? I've heard a lot when
> nearby the main yard with a lot of crews or a crew
> talking with the DS, but at other times while up
> in the mountains and despite being at the same if
> not the exact level of the ROW, the conversation
> seems to one-sided AKA only hearing the DS
> talking.
>
> If there is only one or two hillside
> towers/repeaters in the area or the surrounding
> area, it the signal between the DS and the crew
> more "line of sight" due to FM "straight line
> signal" frequency and any radio or scanner out of
> this "line of sight" is only going to hear the
> dominant side ?
>
> What gives ??

Time of day affects my reception daily. I can hear alot further on my outside antennas during the night, than I ever could during the day. And, when the "band opens", of course I hear even further. So, yes, time of day matters. I guess it depends on your location.

As an example, a BNSF hot box detector 18 miles from me. In the daytime, I cannot receive it at all. At night, it comes in almost half scale on the S-meter. This is a consistent, daily difference, not just occaisional skip or temperature inversions.

(f.y.i.---I'm using Vertex VX-150s for receivers, each on an outside VHF base antenna 25 feet off the ground.)

L.G.


K3HX Wrote:
-------------------------------------------------------
> Pretty much agree with the points made by the
> posters although I've
>
> not found weather or time of day to be a
> significant factor in routine,
>
> day-to-day reception of VHF signals.
>
> Be Well,
>
> Tim Colbert K3HX

Many times I've heard crews a certain distance away fine one minute, then they move a few hundred feet and can't be heard at all. I've also heard crews much farther away clearly, but one much closer very poorly. Just one building can block out at mobile radio if it's in the right spot. The DS transmitter is much more predictable and stronger most of the time. Unfortunately that's just the way it is.

Dave
www.DPDProductions.com
Makers of the "TrainTenna" Monitoring Antenna

rrnut_99 Wrote:
-------------------------------------------------------
> Time of day affects my reception daily. I can hear
> alot further on my outside antennas during the
> night, than I ever could during the day. And, when
> the "band opens", of course I hear even further.
> So, yes, time of day matters. I guess it depends
> on your location.

> As an example, a BNSF hot box detector 18 miles
> from me. In the daytime, I cannot receive it at
> all. At night, it comes in almost half scale on
> the S-meter. This is a consistent, daily
> difference, not just occaisional skip or
> temperature inversions.

This description is typical for VHF rr band signals that I've experienced over the years, but I disagree as to your conclusion. The term "Skip" is non-sequitor and applies to HF signals affected by the ionosphere. Ducting however is a product of conditions within the troposphere. Temperature inversions are not "Occasional", rather they occur frequently, the larger most stable ones being produced by moving air masses. But they are equally influential and prominent during the night hours(the suns heat tends to washout the effect, directly radiating heat evenly through the atmosphere)temperature inversions can form when radiant heat is released by large ground objects such as bodies of water, cities, etc., into a still air mass, which most frequently occurs at night in the absence of air mass systems, fronts, or windy conditions. Inversions are very common, frequently small scale events. The largest scale inversions tend to occur during summer months, August in particular, but they occur throughout the year, as the key component is: air in layers of differing temperatures. That is not to say that the multitude of variables as listed by wa4mur and others above don't apply, clearly they do because of the nature of VHF's line of sight propogation characteristics. But for VHF, if you have a fixed listening rig(as you described), and a fixed transmission source of constant signal strength(as you described), then the air between them is the only variable.


> K3HX Wrote:
> --------------------------------------------------
> -----
> > Pretty much agree with the points made by the
> > posters although I've
> >
> > not found weather or time of day to be a
> > significant factor in routine,
> >
> > day-to-day reception of VHF signals.
> >
> > Be Well,
> >
> > Tim Colbert K3HX

I've always considered monitoring rr band to be a slightly different animal than the typical VHF systems used, for example, by Police, Fire, Med, and other business users, in a basically confined setting of trunked, repeatered systems. Different at least in terms of what the typical railfan monitoring the band is trying to accomplish. To be sure, rr band monitoring can be viewed the same as those listed above, in that one may be listening to, and primarily for, signals within say a large city. But rr monitoring often can fit the description of sitting trackside and trying to grab transmissions from one direction or other from as many miles away as possible. At least to me, there is more than a subtle difference there.