Hello again !
*GOLDEN NUGGET* A tuned aerial uses less power !
Let’s get back to fitting a ground plane and see the effects. We are aiming to get the frequency to mid air band 127Mhz and 50 ohms and SWR below 2.
I have now cut four 700mm lengths of wire to fit as a ground plane and ready to solder to the base of the BNC connector.
Now that I have soldered the first leg I think it might be a good idea to take a quick measurement. Aerial 633mm Ground plane 700mm 60 degrees.
As you can already see a SWR of 1.05 is to dream of. We can change the frequency by adjusting the aerial length later.
This proves the point from the previous blog that the ground plane makes a HUGE difference and I will accumulate the results of various configurations to build a clearer picture.
First the other three ground plane wires will be added to the base of our antenna using standard household electrical wire and of course some solder. The idea is to test the four ground plane wires with the angle down from horizontal set to 0-20-45-60 and at various lengths and aluminium foil covering the ground plane at 0 degrees. Not done at 20-45 or 60 degrees.
Our antenna, the blue wire, will be held vertical to the horizon as good as I can without the aid of a spirit level or such. Please also note that the angles are also all estimates as doing the job accurately and scientifically doesn’t need to be done, just like re-inventing the wheel. This is intended to be informative and show how an an antenna behaves and to get it to its full potential. Horizon to horizon.
These are the positions of our aerial that will be used.
Aerial 633mm Ground plane 700mm
Looks like the best combination for the antenna/ground plane relationship at the moment is at 45 degrees.
*Golden Nugget * The angle between the aerial and its ground plane play a major role in its performance.
Here we add silver foil to the ground plane (0 degrees) to simulate a metal fuselage
Here we can see that the best SWR reading we get is with 50%.
Now maybe you can see why I measured the single ground plane wire and got a good SWR reading. I know where we are heading and the tests up to now already point in that direction.
*GOLDEN NUGGET* An antenna with its matched ground plane does not need to be swung or retuned again.
You can totally destroy the antenna performance (doughnut) if you place or have a metal or magnetic object between the ground plane and the aerial. More on that later.
Next we start cutting down the wires. First we will bring the ground wire down to the same 633mm of the aerial and then work on tuning the blue aerial wire. Of course our aircraft are machines of the sky and are all different to name a few. Just like people all aircraft are individual and have this and that all dotted around. Like swinging an aircraft compass an antenna needs to be tuned on the airframe and adding or removing items that influence the antenna especially if you do not have a matched ground plane independent of the airframe. Later you will see why it is easier to to isolate your antenna from the airframe grounding. (this is a biggy!)
Cutting the antenna down to size
We have now cut the ground wires to the same as the antenna in two stages and measured the various angles of the ground plane, 0-20-45-60 degrees. From 700mm to 667mm and then to 633mm to match the blue aerial wire.
Then we will cut the blue antenna wire in stages from its 633mm to 540mm. Best to stop at 540mm as we know our mid range air band frequency that we want to achiev is 127 Mhz or im they region.
Aerial 633mm Ground plane 633mm
You can clearly see how the angle between the aerial and ground plane are interacting. Best SWR at 60 degrees at the moment.
Aerial 633mm Ground plane 633mm
Now that the aerial and ground plane are the same length we will start cutting the aerial down to size.
Ground plane 663mm 60 degrees
We can see how the frequency AND the ohms are slowly working towards our desired mid aviation band of 127 Mhz and 50 ohms. We also have the choice to tune our aerials to 121.5 the emergency frequency if we want. We do have to remember that moving from the mid airband frequency of 127 Mhz that it can reduce the performance of either the upper and lower frequency depending on your selection. Generally it is better to tune mid range but it could be due to your location you want to tune the aerial to an exact frequency. eg. in the middle of Africa or wherever it is isolated. Airfields can also tune their antennas to their specific allocated frequency and then add power, you will hear them from miles away, especially if you have a high wattage radio in your plane and your antennas are matched your coms will be at their best, or at least your aerial is. Horizon to horizon and strengthfive.com
Generally we as pilots put up with the coms we have as we just enjoy the feeling of flight and not waste time fiddling on the ground with those silly wires.
OK. Let’s start trimming down that ground plane.I have the positions drawn on the ground plane wires ready to cut off and then we will adjust the aerial accordingly.
Aerial 540mm Ground plane 60 degrees
Ground plane 600mm
Ground plane 558mm
Ground plane 540mm
I adjusted the degrees of the ground plane to reduce the ohms and we have now ended up with a perfectly tuned antenna. Aviation centre band of 127 Mhz and 50 Ohms. As good as it gets for now but still be improved, we will get onto that later in the Balun Adventure.
Both Aerial and Ground plane are the same 540mm, Ground plane 50 degrees and we have 127Mhz and 50 Ohms(Z).
*GOLDEN NUGGET* When you modify the angle between the antenna and the ground plane you modify the ohms!
We hope you have enjoyed the Household antennae/ground plane Adventure with us. In my experience bad coms increases the pilot workload. Not only that when the stress is high, the first thing that degrades is you hearing Ask Boeing and such like, they have done many tests on human behaviour when in emergency scenarios. Good coms and average coms make a great difference to pilot workload and that is my opinion and experience. With this information if you are installing or maintaining an aircraft you will have a better understanding of the antenna and ground plane relationship.
I do hear some of you say but my radio works fine. Yes it might be but your antenna, headsets and earthing in your plane can be straining the system, and yes, you can just turn the power up, but that is sweeping the dirt under the carpet so to say. It is always best to get things running efficiently which makes them last longer, uses less power, improves your enjoyment and brings that precious winged machine to life. If your installation is perfect then good on you and let’s all hear about it. They do exist and you may have heard them on the airwaves already.
Let me just finish asking you a couple question or two. How many headsets have you got and how many do you need ? Basically if your coms are running like they should then one headset would be my answer.
Enjoy every day.