I am looking to make a digital swr and qrp meter for my CB radio. I got tired of tuning my antennae and want to make something that will get the Standing Wave Ratio (an analog number from the meter) and adjust the QRP to obtain the maximum range for the antenna and adjusting the SWR to acceptable range.
This schematic includes the SWR meter and the basic QRP adjuster... I just need to be able to adjust the Capicitor C6 to different levels to tune the antenna.
Any ideas how I can accomplish this?
You will need some ways to measure signal strength - maybe your tuner can output an agc signal.
You will need to driver the motor in both directions - a H-bridge.
You will need a user interface, either to rotate the antenna until a strong-enough signal, or to turn the antenna to a present angle - in the 2nd case, you probably need a way to measure direction (optical encoder or some kind of compass).
QRP is radio slang for "low power" it is not a thing you adjust.
From the look of that circuit all it is doing is switching a capacitor across the antenna. You will have to use an RF relay to do this. Do you want just two adjustments or more?
The project can be done in two stages, detection of SWR and adjustment of the antenna tuning. Basically what you need is a line pickup on the antenna feed, this can be done by inserting a wire into the holes in the insulating core of air spaced coax. Have a look at the ham radio sites for this.
DarthSnarkey:
I just need to be able to adjust the Capicitor C6 to different levels to tune the antenna.
I suppose there must be such a thing as a digital capacitor. If one side of the capacitor is grounded, you could probably make your own using a capacitor ladder, much like a digital resistor.
I suppose there must be such a thing as a digital capacitor.
Not really.
There is a varactor, a voltage controlled capacitor, however I think the best way to to this would be to put a stepping motor onto a moving vane variable capacitor.
I'm a little confused as to how to go about this. I need to adjust the "low power" to be the most efficient for my antenna. I wanted to avoid a capacitor array but if it would work the best I can go that route. The SWR meter feeds directly into the arduino analog pins then based on the result of the poll it adjusts the qrp capacitor until the swr reads 1 (for channels 1 & 40) or 2 (for all other channels). I drive tractor trailer but am always moving from truck to truck so manually tuning the antenna is a waste... I want to automate it
I need to adjust the "low power" to be the most efficient for my antenna
No, that does not make sense.
it adjusts the qrp capacitor
The term low power capacitor is totally meaningless here.
What you want to adjust the the impedance matching circuit between your transmitter and the antenna in such a way that maximum power transfer is achieved by making the output impedance of this matching circuit match the impedance of your antenna.
This involves changing the values of the inductors and / or the capacitors in this matching circuit. The SWR meter will give you a rough idea when this matching point is reached. In fact professional broadcast engineers do not tune a transmitter for minimum SWR but for maximum power out, they are not quite the same thing.
It is probably easier to alter the capacitance than the inductance although if you take the bank switching approach you can do this with multiple relays.
[/quote]
I suppose there must be such a thing as a digital capacitor. If one side of the capacitor is grounded, you could probably make your own using a capacitor ladder, much like a digital resistor.
[/quote]
What is being discussed here under the slang "QRP Adjuster", is an automatic antenna tuner, and a variety of such devices for various applications are commercially available. It is not at all unreasonable to "roll your own" and research on the more meaningful term "automatic antenna tuner" will reveal plenty.
This is however, in reference to RF transmission, which is to say RF power of at least a few watts. "Digital capacitors" have no relevance here, they are for an entirely different purpose. Varactors are not suitable as they are actually used to deliberately generate distortion as frequency multipliers - something you definitely do not want on the output of your CB transmitter.
To tune properly, you need a wide capacitance range, and the simplest way of doing this is to use a variable capacitor attached to a stepper - or possibly a servo. The "serious" automatic tuners for decent power of course use (small) geared DC motors to drive larger variable inductors. Relays are also used, but must be suitable for RF use - short contact arms and some shielding.
Varactors are not suitable as they are actually used to deliberately generate distortion as frequency multipliers - something you definitely do not want on the output of your CB transmitter.
No one suggested they were suitable for this application.
Varactors are not suitable as they are actually used to deliberately generate distortion as frequency multipliers - something you definitely do not want on the output of your CB transmitter.
No one suggested they were suitable for this application.
You did not seem to clearly exclude them:
Grumpy_Mike:
There is a varactor, a voltage controlled capacitor, however I think the best way to to this would be to put a stepping motor onto a moving vane variable capacitor.
