G0OIK - Aerial Construction

These are just some pieces of information regarding aerial construction that I have stored here for future reference. I hope you also find it of some use!

160m aerial construction
Notes taken from VARC General discussion forum:
With many thanks to G3YYD, M0MMI, M0TAZ and G6CMD for this content!


It is easily stated how to make the most of a short (in wavelength terms) bit of wire.


1. Make it as long as possible but make sure the far end is spaced well away from anything that will absorb RF - trees, wet brick work etc. The shorter the antenna the higher the voltage is on the end and the higher the potential for absorption. 6 foot from a tree is about the minimum for the ends of the antenna.

2. The best loading is to make it appear longer by adding more capacitance to the end. This can be done by multiple wires that spread out as far as you can. These are ideally at right angles from the main antenna run but anything is better than nothing as long as it does not run back in parallel with the main antenna wire. So a dog leg antenna is not a problem.

3. The next less efficient method is adding a loading coil between the end capacitance wires and the rest of the antenna as this results in the next highest radiation resistance. Middle or centre loading is not as good and do not put a loading coil near the feed point of the antenna unless it is just a small value to help match the antenna.

4. The loading coil if used needs to be heavy gauge wire and the shorter the antenna the heavier the gauge to the point that small bore central heating pipe would be useful. I would suggest 2mm diameter is the minimum. The former of the coil needs to be large diameter for minimum loss and have low losses itself.

5. Ensure you cover as much of your garden with copper wire radials leading back to where the feeder goes to the antenna. A single earth stake is a waste of time. The shorter the antenna the lower the radiation resistance the higher the current is and the losses in the ground go up by the square of the current. Put 20% of your physical effort into the wire above ground with 80% into the ground screen and it will pay dividends. For temporary ground can use chicken wire laid over the grass but not for permanent use as within a few months the acids in the soil will dissolve the zinc and the iron wire will oxidise to rust. There is no substitute for copper wire preferably plastic coated except for silver wire plastic coated but if you are rich enough to afford silver wire then you will not have a small garden!

6. If you get it working well with a short antenna length then the VSWR will be narrow so will need either an auto-ATU or something you can adjust quickly. If the antenna is short and a wide VSWR bandwidth then your antenna/ground have very high losses which equals poor transmit signal strength.

Ground plane
If you have a lawn then a spade to make slits into the grass plus some wire and the radials can go under the lawn. Ideal time of year to do this as ground is soft so the effort is low and by early spring no sign of the slits will show. Lay the radials by starting at the far end from the feed point. For 160m the ends of the radials need to be spaced by about 2.5metres. Any closer is a waste of wire and any wider increases the losses. But if you want to use the same radials on 80m then 1.2m is the spacing to use and pro-rata for the higher frequencies.

Even short radials of a few feet is better than no radials. Obviously if you have the room then longer radials are better as long as the end spacing matches the above paragraph.
73 David G3YYD

Another option is to look towards a loaded vertical. A 12m roach pole, having an inductor to give 265 uh. This is a little high Q but will allow me to position the coil around 3/4 up the pole. This gives me around 8m below the coil and around 4 metres above.

I can then feed with coax and it presents 50 ohms at that frequency. I often use it for either psk or jt65 and so don't require a great bandwidth.

I have a metal fence at the back of me that's a 100m or so long that I ground it to...
It seems to get out OK, but for me the problem is a very high noise floor.
Its often s9 or more in the evening.
Just another tool in the bag.
Dave
M0TAZ

Dave
The 50 ohm you are measuring is mainly due to all the losses. An antenna that tall without top loading will have a radiation impedance in the region of 2 or 3 ohm. This means your radiated power is about less than a 100th of the input power.

Run some wires from the top of your roach pole to some suitable anchoring place as far from the pole as you can manage. This will increase antenna efficiency by a considerable amount as it will raise the radiation resistance yet the loss resistances will stay the same. You will need to remove some turns off the pole to compensate for the increased capacitance of the top loading wires. Less turns on the loading coil will decrease the loss resistance of the coil and again improve antenna efficiency.

I measured the noise on my 160m T antenna (68ft high with top loading wires) with a good ground screen and it measured -33dBm in 2.4KHz bandwidth. Assuming S9=50uV that is S9+40! Someone had a new toy in the neighbourhood, but as I took the mast down a couple of weeks later for moving QTH did not bother to find the QRM source. However before that noise was typically in the 9 to 9+10dB region. This antenna was heard in VK6 (Western Australia) at 579 but I could not hear him unfortunately. Further when it rained as the antenna was only 350m from 400Kv power lines 160m was impossible due to the corona noise of the power line.
73 David

Thanks Dave that's interesting.
Something I don't quite understand, with the loading coil, the antenna is presenting 50 ohms because it's working like an electrical shortened 1/4 wave.

I have tried this setup on both a 12 and 18m poles using different coils.
My understanding is that the wire above the coil is already very high impedance, in a similar way to a capacitive hat.

Why should this be any less efficient than any other shortened antenna?
Wouldn't the high Q be a good thing, in effect presenting a tuned circuit and providing a high rejection to out of band signals / interference?
Thanks
Dave
M0TAZ

Dave
The impedance you are obtaining is the radiation resistance plus the antenna loss resistance (mainly the loading inductance) plus the ground loss resistance. Add together this comes to about 50 ohms. This will comprise mainly loss resistance as a 12m high centre loaded vertical on 160m has a radiation resistance of 4 ohms. The loss resistances total 46 ohms. The best book I know on vertical antenna is ON4UN’s Low-Band DXing, from which I got the radiation resistance figure.

The power going into the antenna is divided between the radiation resistance and the loss resistances. The radiation resistance is the bit you want and the loss resistances dissipate the RF as heat.

To increase the radiation resistance and reduce the loss resistance the simplest way is to add top loading wires. This will add extra capacitance to the antenna so the loading inductance can be made smaller which in turn decreases the loss resistance. It also increases the antenna radiation resistance. So radiation resistance goes up and loss resistance goes down QED radiated power goes up. With all top loading and no inductive loading the radiation resistance increases to 9 ohms for a 12m high vertical and there is no coil loss.

So how much better is the TX signal? Let’s assume the centre coil was perfect with no loss so the 46 ohms was all ground loss. Then power is now divided between 9/46 in place of 4/46 so the signal is 6dB stronger. Take into account real coil loss then the signal is probably 10dB stronger. The signal can be made very much stronger for the cost of some top loading wires.

The next thing to do is decrease the ground loss resistance by adding radials even short ones of a few feet will make a difference. The current in the ground is highest at the antenna base and decreases away from the base. As power dissipated is proportional to current squared short radials near the base make a lot of difference to antenna efficiency.

If you get all of this right then the antenna will have a much lower impedance than 50 ohms.
A top band small vertical antenna that has a good VSWR is a high loss antenna.

Going to 16m top loaded vertical the radiation resistance becomes 13 ohms, centre loaded is 7 ohms and a base loaded is 4 ohms.
73 David

Hi Dave,

One key thing to consider is 'where's the current?' As current is what makes the electrons move, which is what we want.

Putting a coil at the bottom is convenient. This takes the high impedance end and turns into a low impedance. High impedance = low current. If you put the loading at the other end - such as a capacity hat - keeps the high current part of the antenna radiating and the low current part is dissipated in the hat. Challenging to build properly.

The loop gives you common mode interference rejection, so is best for electrically noisey environments such as the loft.

The Q of an antenna gets quite sharp at lower frequencies - sub 3k bandwidth is not unusual. You also have to watch those peak voltages and currents in your ATU.

The 198kHz BBC antenna at Droitwich is a capacity hat antenna with a large diameter to improve the Q of the circuit. Despite all this the input impedance is fractions of an Ohm (resistive) and with 9kHz transmit bandwidth has an ATU which is sloped across the channel to give the transmitter a sporting chance to hit the bandwidth without destroying the ATU!

73 Chris G6CMD