Wire antennas for listeners
Many beginners, novice listeners or just licensed hams, ask regularly information about the installation of an HF antenna in their studio or better, in their attic, in the garden or or top of their roof. These amateurs want to hear HF bands but not all have the know-how and are sometimes stucked in trying to install their antenna. The questions to which we are going to answer are what aerial to choose to get a good signal, where and how to install it ? For active hams theses questions may extend to the effects of some materials and soils on waves propagation.
These few problems are of importance in HF bands as their wavelengths measure between 160 and 10 m long... and theoretically require an antenna as long, or almost.
At first sight, browsing antenna catalogs, there are hundreds of models available. On one side this is a good news as these aerials come in virtually an infinite number of models and must, theoretically, offer a solution for each "problem" or usage. But on the other side, the choice is so hard that we need advice to select an antenna suited to your needs. If we can discuss without end about this subject - ARRL and RSGB have each published around twenty books about antennas - some key points must be highlighted.
Receiving is not transmitting
Theoretically one tells that an antenna is reciprocal, meaning that what is "good" for receiving is also "good" for transmitting. The hams' "Bible", the ARRL Antenna Book, writes the same comment but tempers this sentence in speaking of pick-up efficiency for receiving.
Indeed as we introduced in discussing about Basics of antennas, an antenna dedicated to only receive ham emissions does not have to be as "perfect" as a transmitting antenna, and still more, it "works" not at all in the same way as in emission.
Let's take an example. I still remember very well that when I was listener I used an unmatched, non-harmonic (aperiodic) long wire tight horizontally in V over the roof and connected to my receiver to listen to DX stations. I captured this way emissions that came from the Kerguelen or Marion islands and even the weak signals from stations located in the middle of the Pacific or in Australia, 16000 km away ! But I should have been unable to work the same stations due to a very bad SWR quasi infinite...not the single watt might radiate from this "antenna"; good for receiving but a nightmare for transmitting !
So, even if that sound like a contradiction, a receiving antenna is not as critical in some respects as a transmitting antenna : even if the nature of your long wire is not appropriated (e.g. coax), if its length, its height above the ground and the feed line characteristics are roughly calculated, you will hear enough stations to be occupied with your receiver all your life long ! In the best case, once licensed, if your long wire is well tuned without too much losses you could even transmit in such conditions and work hams located 10000 km away or more. Some hams call that a Field Day activity, Hi ! But forget this foolish idea and let's use to good practices, and study this problem.
For emitting, an antenna should be efficient, I mean that all the input power supplied to the antenna should be radiated with as few losses as possible. This assumption applies to all antennas, that they are designed to work on HF or on any other band, and all the more at low power (QRP). A very small VHF Yagi for example should be thus as efficient as a HF beam 10 times larger if we can ensure it a high gain at high frequency (the pickup efficiency decreasing indeed as the frequency increases, what can be improved in using Yagi's with still more directors).
On the contrary, in receiving the pickup efficiency put at the disadvantage of listeners because the physical length of the antenna directly influence the amount of energy that it is able to pick up.
For receiving, an antenna is neither exactly passive nor really active. The main problem is the antenna ability to pick up passing waves near conductors. The most important is the physical size of the antenna used, then its radiation pattern.
Let's imagine a HF antenna resonant on the 20-meters band. The 1/2l dipole radiator (driven element) is 10 m long or so. The equivalent long wire is twice as long. These conductors are able to draw the energy from a distant station to your receiver because they have the property to attract electromagnetic fields of same resonance, in our case up to a distance of about 1/4l or 5 m away from conductors. The radiation pattern helps then to receive the signal from the transmitter where it is the most strong in both azimut and elevation.
Now compare these properties to the ones of a small VHF Yagi : its largest radiator has a wingspan of 1m instead of 10 m. Not only it is 10 times smaller than the HF beam but the coverage of its radiator reaches with difficulties a distance of 50 cm... With its 20 elements, this small Yagi offers however a high gain compared to a dipole. But this small "plus" in directivity cannot fight against its overall sizing compared to the HF beam...
Indeed, the energy carried by waves are travelling evenly in space regardless the wavelength. From then on the effective area that the receiving antenna can use is not higher than the square of the wavelength, √l. That means that in our case, flood in an electromagnetic field of the same strength, the VHF antenna will capture 100 times less energy than the HF beam. This value exceeds 50000 times if we compare an UHF to a low band antenna !
Worst, when impedances are well matched between the antenna and the receiver only half of the total power picked up by the antenna from passing waves is transfered to the receiver, the other half being reradiated into space.
However, and this is a good news, the gain of both antennas is not affected by the frequency and both systems are able to capture the same amount of energy in respect with their specifications. At least on this point they are on par
The rain and soil influence on antennas
This problem is not very important for listeners. But for purists, those who appreciate much to work in condition as good as possible, this factor is not futile and can affect the radiation pattern, and thus the pick up properties of your antenna. And of course for licensed hams these effects will affect your "angle of fire".
Losses in an antenna due to the ground, without speaking of the feeder and coaxial, are of three kinds:
- If the soil in made of sand, the ground is a poor conductor, excepting if it is wet;
- Deep soft soil like agricultural field is conductive;
- Water, from lakes or the ocean is very conductive.
So, if you are searching for the best soil to erect your antenna and DXing, the rain falling on the ground yield a mitigated effect, sometimes negative in specific directions (on dipoles).
This has not much effect on an horizontal antenna displaying very low angles of takeoff (say below 20°). But conversely, your vertical will work or not work at all depending on the way that the underground absorbs or reflect RF signals emitted by the antenna. Therefore 3 raindrops falling on each square cm cannot much influence them.
With a directive antenna you must well understand that this is not solely the layer directly beneath the antenna that matters, but the nature of the ground up to 10lin front of your antenna if you are searching very low angle over the horizon.