Problems with RF transmitter/receiver

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi All,

I'm having some problems with the steadiness of my RF transmitter.
Here is the product I'm using: http://www.robotshop.ca/on-shine-high-sensitivity-tx-rx-1.html

I'm transmitting through one brick wall and one window at about 20-30 feet. It seems to transmit fine if my body is nowhere near the line-of-sight but the second I stand even just off to the side it no longer receives. I tried upping the voltage to the transmitter with no luck. I checked the antenna and it's the right length for the frequency I'm using. Any thoughts on this ?

Are there any people out there that have had success using these low-cost components ?

Thanks in advance,
Mark

2

Problem is that a person is just a lump of water as far as RF is concerned. A lump that is absorbing the RF energy.

Changing the voltage will not up the power to any great extent. What you can do is to make sure the antenna are correct and matched. That is the polarisation of each is the same. So if you have a quarter wavelength vertical it is pointing up on both transmitter and receiver. The ground plane is also important make sure that is good.

Probably the best bet is to try and make some sort of direction antenna at both the transmit end and the receive end, but at the end of the day you are dealing with very low powers that are easily absorbed.

3

Hey thanks for the info.
I will try to align the antennas the same for the receiver and transmitter.

The ground plane is also important make sure that is good

I think my antennas may not be wired correctly with the ground plane in mind. Can you explain how to wire the antenna with the ground plane in mind ?

Thanks again !

Mark

4

Have a look at this:_

As this radio operates very close to the Ham band of 70cm look at Ham radio antenna design sites as well.

5

Thanks for the info.
I have yet to look into Ham radio antenna design but by reading the Wiki let me understand if I get this right.

I now understand how a ground plane antenna works. Space is an issue with having the radials coming out at four corners. I will eventually be mounting this module inside of a wall.

Mark

6

So if you can't use a quarter wave with a ground plane then you will have to go for a half wave dipole.

7

So if I want to go with a vertical whip so I connect the ground-plane to the ground of my circuit ? Or does the ground-plane just sit on it's own ?

Mark

8

The ground plane must be part of the transmission line connecting the antenna to the final output stage of the transmitter. It has to be part of the circuit but it's not sufficient to just tag a wire from it to the ground of your system.

However, think of a ground plane on a quarter wave whip as a mirror reflecting the upper half of the antenna.

A half wave dipole is basically a quarter wave whip end on with a quarter wave ground, nice and slim.

9

Oh ok so I must have my vertical whip connected to the ground-plane physically ?

I understood that the ground plane completed the circuit via connecting to the ground on the circuit.

In many ground-plane designs I've seen the ground-plane NOT touching the vertical radiator. The most common design involves 4 radials which would then be connected to the shielding of a coax cable and on many threads I have read that the vertical radiator must NOT touch the shielding of the coax. This true ?

Thanks,
Mark

10

I must have my vertical whip connected to the ground-plane physically

No stick and wrong end here.

The ground plane is physically connected to the other side of the transmission line, normally at ground potential.

I have read that the vertical radiator must NOT touch the shielding of the coax. This true ?

Yes if you want it to work.

11

OK this makes sense now.
Thanks