Introduction: When I get on the air invariably I am a Portable or Pedestrian mobile Station.I usually set up in parkland or from the edge of the ocean. These location afford much lower noise levels then in suburbia.
In many cases stations are amazes at my signal strengths both local and DX, 5 by 9 reports are common place all over the world. I get many enquiries on my station equipment especially the antenna. My answer is simple, I report I am using a ¼ wave vertical with a ground tuning unit or GTU. In most cases operators simply don’t understand what a GTU is or how it works and ask for an explanation. To try and explain on the air is simply impossible so below I have prepared a simple explanation on what a GTU is and how it works.
Credits: I was introduced to the GTU by Dave G4AKC who to my knowledge perfected the GTU and applied it to his pedestrian mobile and portable stations. Dave is well known by Stations worldwide as the Pedestrian Mobile from Blackpool UK with the huge signal and the founder of the “Realhfmobile Facebook Group”.
¼ wave Vertical Ground Plane Antenna:
Most operators are familiar with this antenna, there is a lot of information that can be accessed on the net on how to make one and how it works. Many people simply dismiss this simple antenna as being a basic antenna and turn their efforts to other more sophisticated designs hoping for better results. Actually the ¼ wave antenna is a very high performance antenna that is underutilised but is perfect for the portable station application because of its simplicity, low angle of radiation and omnidirectional characteristics. What turns many people off is the installation of radials and the never ending debate on how many radials are required for the perfect installation somewhere between 4 and 120 !
And the perfect length?? Whether the radials should be elevated from the ground or laid on the ground, the downward sloping angle of the radials to obtain the 50 ohm matching impedance.
Imagine having a perfect ¼ vertical with no radials. The advantages to a portable station are huge!
Enter the Ground Tuning Unit or GTU
Ground Plane or Counterpoise?: When we step back and look at a ¼ wave vertical antenna it is in fact a simple dipole turned 90 degrees into a vertical position with one half sticking straight up and the radials or counterpoise making up the other half. In theory the currents in both halves should be equal. The top half current radiating the signal and the bottom half current forming the ground plane. If the ground plane current is insufficient caused by
a poor grounding system the radiated energy of the overall antenna will be reduced.
The Ground conditions
The ground conditions dictate how well a vertical antenna will work as mentioned above the ground forms the “other half” of the vertical antenna.
With this in mind a vertical working at ground level over ideal ground conditions such as very near to the sea will produce far better results than the same antenna working against poor conductivity ground such as sand or rock away from salt water.
When operating close to the sea, the “ideal ground-plane” is already present in that the salt water of the sea provides the perfect medium for the vertical to work against and so the purpose of the GTU when operating over salt water is to provide a very low impedance down to the salt water on the operating frequency.
When operating over poor ground the settings of the ground tuning unit will be different as in this case we are trying to produce a balanced resonant counterpoise for the vertical to work against, and so you can see the GTU works in quite different ways dependant upon the type of ground that you are working over.
When operating close to the water the antenna needs to be over where the sea covers (or as close to it as possible) as the performance enhancement of the sea water diminishes rapidly as we come out of the “fresnel zone”.
As the final radiated pattern does not develop until a distance of several wavelengths away from the centre of the vertical antenna the benefits from being close to the sea are that there are many reflected and refracted signals off the sea water that add to both the receive and transmitted signals as shown in the diagram below:
The GTU: This is a device that replaces the radials or counterpoise and can be tuned to allow the maximum grounding current to flow thus ensuring maximum radiation from the top half of the dipole.
The current in the top half element is maximised by resonating or tuning its physical length.
The bottom half current is maximised by tuning it to resonance by a tuning circuit, the GTU.
A GTU is installed between the normal counterpoise feed point of the installation and a small sheet of metal called a grounding foot insulated from earth by any thin insulator laid on the ground just below the feed point. The RF grounding current flows from the feed point through the GTU and into the foot and is radiated into the ground.
The GTU can be adjusted to resonate the RF ground current for differing ground conductivity.
Basically a GTU is made up of two circuits. A tuning or matching circuit to resonate and maximise RF ground current and a RF current measuring circuit to monitor the current and enable adjustments for maximum current flow.
GTU Tuner or Matching Circuit:
Thisis simply a variable inductor in series with a variable capacitor.
The inductor can be made from a large 2 inch toroid with 20 turns tapped every second turn and switched by a 12 position sector switch or an air wound inductor coil also tapped. Both are very successful.
The Variable air capacitor can be in the order of 320 to 500 uf similar to that found in old fashion radio tuners.
If you are using a metal mounting box both components must be insulated from ground. Plastic boxes are best suited for constructing a GTU.
Below are 2 photos of a GTU. Note the variable air capacitor, the air wound inductor and the 12 position selector switch.
GTU RF Current Measuring Circuit: There are many designs for this circuit. They are all based around a small toroid sensing transformer producing a RF current which is rectified and displayed on a meter. Because we are only interested in a maximum reading calibration is not necessary. The only meter adjustment required is to keep the needle off the end stops. Central reading is good! This is achieved by a 200K linear pot in series with the meter.
Typical Portable Station Incorporating a GTU:
A typical station is a complete unit that can be carried in the boot of a car and can be lifted out and set up in a few minutes. It will be battery powered and have a telescopic ¼ wave resonant antenna made from a telescopic fibreglass pole and a length of wire or aluminium sections. It will have wheels or be light enough to be moved to a suitable set up location.
I use a small aluminium trolley used for carrying a medium sized suitcase with wheels.
All of the equipment mounted on the trolley must be insulated from the trolley!!!!
The foot of the trolley or in fact the entire trolley frame will act as the GTU grounding foot and will have a thin piece of wood mounted under it to insulate it from ground. There is NO direct connection to ground just capacitive coupling!
Typical equipment on board the trolley will include: Radio, GTU, an antenna tuner (ATU) fitted with a SWR and power meter, a battery possibly a deep cycle model, 18 to 50AH works well depending on you TX power. A mount for your resonant antenna and interconnecting cables. You may also incorporate a linear amp, additional battery required!
Setting up and getting on the air:
After constructing your station you will need to learn how to tune it. Set yourself up in a clear area away from any structures and start the tuning process.
It pays to have a pre resonated ¼ antenna for the band you wish to tune but it is possible to tune the antenna with your portable station.
Tune your radio midway on the desired band and switch to low power and AM or FM transmission. This will give you a nice continuous carrier signal to tune with.
The aim in tuning is to get the highest power into your antenna with the lowest SWR and the highest GTU current.
After successfully tuning your station to the best values switch your ATU to bypass, this should give you an indication of how resonant your antenna is and give you the opportunity to change its length to resonance.
After doing this switch your ATU back in circuit and retune, all should be good!
Now you are ready to switch to SSB and get on the air.
An analogue or digital voltmeter connected to your battery is a good addition for monitoring you battery state.
A valuable item in tuning your station is a Field Strength Meter. By setting it up a short distance from your station you can monitor your transmitted signal while tuning for maximum output.
Note: Every time you set up your station you will have to retune it because of differing ground conditions below the foot. You will learn which ground conditions work best and of course if your operating frequency band changes the GTU will also need to be re-tuned to optimise the ground current.
I have found beach sand to be poor but rocks near the waters edge to be good. Wet soil to be better than dry soil. You will see the results quickly when you compare different locations.
Six key points about the “ground plane”
1) The ground plane for any vertical antenna is just as important as the antenna itself and it will dictate the overall performance of the antenna system. It’s the “other half” of the antenna system and the use of a GTU will enable you to couple to this perfect ground plane without any trailing wires.
2) The complete radiated beam only forms several wavelengths from the antenna so a large ground-plane like the sea is ideal.
3) Multiple reflections from the sea in the “fresnel zone” dramatically improves performance.
4) The salt water of the sea creates a perfect ground-plane for the antenna to work against much like a sheet of copper stretching for many miles.
5) Operating near to the sea increases the ground wave and decreases the angle of radiation, this enhanced effect is only truly effective where the tide covers the sand even at “low tide”
6) Location: Operating close to the sea can see a 15-20dB increase to “low angle” receive and transmitted signals compared to locations even a short distance away from the sea.
Good luck with your station, if constructed correctly you will not be disappointed.
When a distant station asks you what antenna you are using you will have the opportunity to try and explain all this over the air, enjoy!