Antenna swr questions

It is a catch 22. You have to find the happy medium. I vaguely recall seeing somewhere else on this forum what the optimal angle away from vertical is, and if you exceed it, you start harming your ability to transmit and receive. I _think_ it's somewhere in the range of 10-15 degrees. Any more lean forward than that, and like MNdriver said, polarization really starts to take effect.

 

I'm a simple man. I have the ability to comprehend complex ideas, I generally just choose to ignore them. Hurts too much. Easier to use short words that most people understand.

 

You take 2 hits to signal when you mount an antenna so that it's facing forward and parallel to the ground. Here's why:

The first hit you take is loss due to polarization of signal. You automatically lose 50% of your signal when you talk or listen to someone that's got their antenna in a vertical configuration.

The second hit is due to the antenna's radiation pattern. Antennas with 'gain' don't actually increase the power of your signal, they just redirect power from the areas where it would normally be wasted. Antennas radiate signal in a sphere - antennas with 'gain' redirect the power that radiates straight up into the sky, and straight down into the ground such that it is concentrated into a 'donut' shaped pattern as it would appear looking straight down on your truck from above. Picture your truck in the center of this:



When you point the antenna forward, you change the orientation of the 'donut'. Now you're radiating signal to your sides, up and down, but very little goes forward and backward - which is where you ACTUALLY want it.



For every 3db of gain, you lose 1/2 of your signal. So for a 3db antenna pointing forward, you'll lose 50% due to polarization, and another 50% of that (in front and behind) due to radiation pattern. Now your 4 Watt signal is effectively reduced to 1 Watt. For a 6db antenna, you're down to 1/2 Watt effective radiated power in front and behind you. And it scales from there.

As a side note, interference from power lines is also horizontally polarized, so horizontally oriented antennas pick up more of this interference.

 

Now, what affects SWR is the TUNING of your coax / antenna circuit. SWR represents the percentage of transmit power that is reflected back to your radio due to the antenna system not being tuned properly. The main thing that affects tuning is the effective length of the antenna. Also, placing the antenna too close to a metal object introduces capacitance and inductance into the antenna circuit (basically turns it into a transformer), and detunes it, therefore increasing the SWR.

Here is a graphical representation of a circuit with a 1:1 SWR - that is, all of the power from the transmitter is radiated by the antenna, and none is reflected back to the transmitter:



Here is an example of a mismatched antenna circuit that will register on your SWR meter:


The bold portion on the right of the image represents the portion of the signal that is actually transmitted. The wave that travels back along the left side of the line represents the portion of the signal that is reflected back to the transmitter.

Keep in mind that you will only have a 1:1 SWR at ONE frequency. The more you tune away from that frequency either up or down, the SWR will increase. This is normal, so tune for SWR on the frequency that you plan to use the most (probably ch. 19).

In case you're wondering, I hold an Advanced class Amateur Radio license, and have worked as a radio broadcast engineer back in the day.

Hope this helps.

 

NOOOOOO Here we go again

Coax does not TUNE anything UNLESS IT IS A DIFFERENT IMPEDANCE ,,,,

If you cut coax the TUNE anything your antenna is BAD .....

 

You and I know that, but I was trying to simplify the explanation.

Some drivers will attempt to cut and terminate their own coax cable, (shorter is better, right?) and usually the terminations are done improperly. This causes problems with SWR as well.

 

Yep and lets not forget the PL259 is NOT 50 ohms .... This is why they are not widely used at abovw 300 meg ... BNC/N are much better but cost $$$$$

 

And try to find an 11m antenna that doesn't have an SO-238 connector!