Railfan Technology > Positioning TrainTenna on magnetic base

I just received my first mobile TrainTenna, which came without instructions.
Is it recommended that the 1/2 whip be inserted all the way into the base (tip making contact with the base) or locked in with a small gap between the tip and base?

Thanks!

Andy

.All the way in.

It should go all the way into the base and then be secured, probably with a set screw.  1/2 wave???  I would guess it's either a 1/4 wave or 5/8 wave.  If it's about 18 inches long it's a 1/4 wave.  If it's about 42 inches long and has base with a loading coil it's a 5/8 wave.  A half wave mobile antenna is somewhat rare due to matching characteristics.  The tip of the 1/2 wave antenna is at a very high impedance point and hard to match to 50 ohm coaxial cable.  If it's fed in the center, it works great but mechanically, it's more complex.  Matching is not quiet a serious matter for receive only but is very important when transmitting.  You're not going to be transmitting on it but a miss match reduces the efficiency of the antenna.  Hooking a 50 ohm cable to a >1000 ohm antenna feed point is like putting a bicycle tire on your tractor.  

I have found that some of the documetation in the DPD adds (Traintenna) have had some errors.  

John

Some folks like 1/2 wave, but it doesn't make a lot of sense on paper. There's also been a number of discussions about J-poles. Also consider that using an antenna for receiving is not the same as transmitting, a receiver works very well with a high impedance antenna. We do use the reciprocity concept to demonstrate beam patterns, but modern receivers don't need the 'power' transfer.

After a while I realized the differences in reported performance is due to the Omni pattern viewed from the side (rather than from above), or where the gain lobes actually touch where the train actually is. In an analogy, the improved antenna gain is simply moving the radiated area (EIRP if transmitting) from directly above the antenna closer to the ground. It's obvious in three dimensions, but not so obvious in two dimensions. Considering a link from an airplane to a ground station might be a better analogy, but the modern transmitted power levels obviate any need for that.

So it's not that the gain pattern of the 1/2 wave or J-pole extend further from the center, it's that the antenna and the transmitter are not at the identical elevation and happen to coincide at a point of high gain (dBi of course).

Did a quick Google and found an interesting visual analogy:

http://www.hamradio.in/circuits/radiation_pattern.php



Edited 2 time(s). Last edit at 04/29/15 16:51 by TCnR.

TCnR Wrote:
-------------------------------------------------------
> Some folks like 1/2 wave, but it doesn't make a
> lot of sense on paper. There's also been a number
> of discussions about J-poles. Also consider that
> using an antenna for receiving is not the same as
> transmitting,

Kind sir, you have been misinformed.

The only time this is true is when the antenna must handle a considerable
amount of power.
For example, an antenna used for high-power transmitting will be constructed
so as to deal with increased current and voltage but the pattern it will display
will be identical on both transmit and receive.  The reciprocity law dictates this.

a receiver works very well with a
> high impedance antenna.

For proper operation, the impedance of the receiver should be identical
to the impedance of the antenna, and the impedance of the feedline (coaxial
cable in most cases) should be the same as the antenna and the receiver.  In
typical RR enthusiast use, these would all  be 50 ohms, refered to as "low impedance."

>We do use the reciprocity
> concept to demonstrate beam patterns, but modern
> receivers don't need the reciprocity.

It is not a matter of need.  Reciprocity is a law of physics.

> After a while I realized the differences in
> reported performance is due to the Omni pattern
> viewed from the side (rather than from above), or
> where the gain lobes actually touch where the
> train actually is. In an analogy, the improved
> antenna gain is simply moving the radiated area
> (EIRP if transmitting) from directly above the
> antenna closer to the ground. It's obvious in
> three dimensions, but not so obvious in two
> dimensions. Considering a link from an airplane to
> a ground station might be a better analogy, but
> the modern transmitted power levels obviate any
> need for that.
>

A better analogy.  For this example, I'll use an vertically-polarized omnidirectional
antenna in a field free from obstructions.  The pattern can be thought of  as taking
a ball of dough and smashing down on it so it becomes less tall but greater in diameter
Going from a ball of dough to a pizza crust.  The same amount of dough is used but the
distance "out" is increased as the distance "up" is decreased.  Antenna design
manipulates the way the "dough ball" is shaped. 

> So it's not that the gain pattern of the 1/2 wave
> or J-pole extend further from the center,

Actually it is.  To get omnidirectional gain, the energy from the high-angle lobes is
typically suppressed and directed towards lower angles. (smashing down the ball
of dough)

By the way, the "J-pole" is a flawed design.  I have never seen one used in a commercial
installation.   Their popularity is due to ease of construction and a faddish, anecdotal  popularity
absent any scientific basis for claims made. 
An explanation of some of the shortcomings of this design here:https://icons.duckduckgo.com/ip2/www.w8ji.com.icow8ji.com/end-fed_vertical.htm
it's
> that the antenna and the transmitter at not at the
> identical elevation and happen to coincide at a
> point of high gain (dBi of course).

Don't understand the point attempted to be made here.

> Did a quick Google and found an interesting visual
> analogy:
>
> http://www.hamradio.in/circuits/radiation_pattern.
> php

Popular understanding of antenna operation is frequently missing a
scientific basis and often contains repeated myths and "old hams'
tales" with bits of misunderstood  techno-jargon mixed in.

I made my living in the electronics and 2-way radio business.  I've designed, tested,
built, repaired, installed, moved, tuned, and modified over 700 antennas in military,
public safety, medical (MRI,) business and residential service. Everything from
microwaves to AM broadcast band.

There is no magic involved in antenna operation, just physics.

As for the original post, call the manufacturer and follow their
instructions.  If you  know an amateur radio operator, they will most
likely have an antenna analyzer and can tune the antenna for optimum
performance.

Be Well,

Tim Colbert  K3HX

No, we're both trying to describe an elephant to someone who hasn't had the joy if seeing, hearing and smelling an elephant.

I don't deal with 1/2 wave, j-antennas, or beam-forming cookie dough, don't blame for that one.



Edited 1 time(s). Last edit at 04/29/15 19:51 by TCnR.

Gee, I thought it was such a straight-forward question.

I somewhat agree with everyone, sort of.  For receiving you can hook a piece of wire to the radio and you're going to hear something.  The problem for most of us is that if the antenna is perfectly matched we're going to hear a train.  If the match is awful, we're going to hear a train.  We usually can't tell much difference.  However, even with the extremely low power present when receiving, the better the match, the better the performance.  When we transmit we move several watts from the radio to the antenna.   When receiving, it's moving nanowatts (or less) from the antenna to the radio.  The better the fit, the better the system works.

And all of this because someone ask how far to stick the whip down into the base on his new antenna.

John

Agree that with a modern receiver the difference in reception becomes very small. The advantages of the mentioned antennas become difficult to differentiate. They still have their own properties, in most conditions they are just less obvious.

I haven't tried TrainTenna products, but I'd say yes, push it all the way in.  Any tuning for the RR band should already have accounted for that by pruning the whip to the proper frequency.   Have fun !