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Considering just electrical properties, the most significant parameter for your selection of antenna conductor is resistance. You want to keep the resistance to a minimum, because when current flows through a resistance, the electrical power is converted to heat, according to Joule heating:
$$ P = I^2 R $$
Any energy you use to make heat is energy you aren'...
16
The primary factor to consider is the directivity of the parabolic antenna. It is given as:
$$d=\frac{(\pi D)^2}{\lambda^2} \tag 1$$
where D is the diameter of the dish and $\lambda$ is the wavelength of operation, both in the same units.
To convert frequency to wavelength in meters, we use:
$$\lambda=\frac{300}{f} \tag 2$$
where f is the frequency in ...
13
Maybe.... A big project and probably not practical starting out. Would make a conversation piece or odd recycling project. How are you going to point it? Smaller antennas may work just as well.
A modern "Dish TV" or "DIREC TV" (R) that is about a foot across is practically useless for amateur radio transmitting and receiving.
Below is a stock picture (...
12
Most antennas are reciprocal — they have the same properties when receiving as when transmitting. This means that in many ways, a good antenna design makes both a good receiving antenna and a good transmitting antenna.
(The biggest exception to this is active antennas which have an internal amplifier that only works for receiving.)
However, the properties ...
11
Because of the skin effect, which causes most of the electric current to be concentrated around the outer surface of the wire at high frequencies, larger gauge wire is needed for RF than for an equivalent DC current. As a general rule of thumb, 16 gauge is sufficient up to 100 watts, but 12 would be ideal. Any system exceeding 100 watts really needs to have ...
10
At power levels that low, you probably don't need an SWR meter permanently installed. You will find it useful to have an SWR meter or antenna analyzer available when building the antenna, though, to ensure it is properly operating on your chosen frequencies. Perhaps you can borrow one from a friend, mentor, or local radio club to use while building your kit?
10
You have aptly discovered why a balun is necessary when feeding a dipole with coax. You are right to think the book is wrong, because it is. With a coax feed and no balun, the current distributions on the dipole are not equal because some share of the current that should be on the right half (connected to the shield) of the dipole is instead flowing on the ...
9
A balun matches a balanced load to an unbalanced line, but it can also do other useful things. A current balun can present a high impedance to common-mode signals, which will help reject noise. Common mode signals are the same on both conductors, so are not "balanced" or differential.
An unun is an impedance transformer, usually 4:1 or 9:1, which matches an ...
9
One way to tell is by its effects. Do you hear a garbled version of yourself in nearby speakers when you transmit SSB? Do GFCI outlets pop even though no one is being electrocuted? Does handling the transmission line give you RF burns when transmitting, or change what you hear when receiving? If you had these problems and now you don't, you must have been ...
9
The canonical solution to this problem is a Wilkinson power divider. There are of course other power dividers, but the Wilkinson is easy to fabricate, provides good isolation between the output ports, and is lossless when the output ports are equal and in phase, which is true for an ideal antenna array.
If you were to make one of these yourself, you'd ...
9
You have an SMA connector – use that! You can buy ready-made coax cables with SMA connectors on both ends.
As mentioned, what you want is a coax cable. The electromagnetic wave travels within the cable, so, no, if not damaged and properly attached to the radio and to an actual antenna, it won't double as antenna.
Very much like your TV cable doesn't emit ...
8
Yes, this works fine. It is often called a "fan dipole".
There are separate resonant dipoles for each band, all fed from a common point. The wires should be separated by a foot or more. If you have enough tie points at the ends, you can run them in slightly different directions.
The resonant dipole will have a low impedance, around 50 Ohms. The non-...
8
Here is an answer for the European hams among us.
Printable copies are available from my web site.
Wire myths
There happens to be a lot of fuss out there about antenna wire. Most amateur radio shops sell "special" antenna wire at unreasonably high price, whilst most of the claims made about these wires are untrue. The situation reminds me a bit of the many ...
8
In short, no.
In long, it's really complicated. The wider your wire, the wider the bandwidth, due to the increase in available paths. Wide multi-strand wire does better against skin effect (more conductive area).
But, the "more exposed area" doesn't work here. You are picking up the E-field with this kind of antenna, so it's the voltages out on the tips ...
8
You should not make any coil in the middle of the antenna as that will greatly change its RF characteristics. Instead, bundle up the wire at the ends in some fashion:
One practical solution is to fold the wire back on itself and fasten the free end onto the main line; then you have a loop you can also use for supporting the antenna, though you might want to ...
8
The antenna itself won't be significantly affected by the water.
However, waterproofing the coax connection is essential. If this is not done, water will creep inside the coax by capillary action and ruin the coax.
There are several products that can be used, for some examples see 3M's application guide. However all have three elements:
An underlying ...
8
In free space, the electric and magnetic fields are always in a fixed ratio, a physical constant called the impedance of free space, about 377 volts per ampere. The two are always in phase (and thus have identical wavelength) but in orthogonal angles. In fact, the magnetic field is explained by relativity to be the effect of length contraction of moving ...
7
You can make the wire as thin as you want. There are two non-obvious things to point out:
Firstly, as you make it thinner, resistive losses go up. You could make a worst-case calculation of the resistive losses by assuming the current is at a maximum everywhere in the antenna. If you put 20W into a 50Ω antenna, the current will be:
$$ \begin{align}
I^2 R &...
7
A Noise Bridge is a very useful piece of test kit, often described as a "poor man's antenna analyzer". It's capable of measuring the exact impedance of an antenna, both the resistive and reactive components, something that normally requires a pricey bit of lab equipment.
Unlike an SWR meter, the noise bridge, requires little or no transmitted power applied ...
7
A coaxial cable is a transmission line by itself, and therefore the velocity factor is known. It depends on the dielectric properties, geometry, and conductivity.
A single wire is not a transmission line. You need a return path for the current, and that will most probably be a second wire somewhere, or a ground plane.
So the geometry and dielectrics around ...
7
I've not personally built antennas from scratch, but I appreciate my antenna analyzer just for being a good instrument — making the invisible aspects of my antenna system visible.
Compared to using a SWR meter for the purpose, an antenna analyzer:
Displays more information.
A SWR meter still gives you enough information, in the sense that you can try ...
7
Besides mechanical differences, the primary difference between aluminum and copper in antenna construction is RF resistance. Copper will have slightly less RF resistance for the same surface area. Increasing the surface area slightly allows aluminum to exhibit the same RF resistance as copper.
RF resistance is unique due to the tendency of the RF current to ...
7
Stainless steel fasteners, lead and tin are compatible with copper. Lead tin solder, however, has poor mechanical strength as you have apparently discovered. Solder can be used to improve conductivity and moisture ingress and is best when used with other mechanical strengthening measures. So the use of stainless fasteners to improve the mechanical strength ...
7
PCB is an appropriate and useful way of making antennas for higher frequencies, say from 900 MHz and up.
My company made thousands of GSM antennas for window mounting (in the days of car-installed cell phones). These were a fan dipole for 900 and 1800 MHz.
Here is a PCB antenna I found on the web, for example:
We also made all sorts of high gain Omni ...
7
One leg of the antenna goes to the center conductor and the other leg of the antenna goes to the braided shield of the coaxial cable. The terminal block is just a way to make the connections, and it has two electrically separate positions. You can use any means of making the connection you have handy, as long as it is small compared to the wavelength.
(...
7
First, a general statement: the antenna analyzer has one set parameter, the frequency, and one measured parameter, the impedance (which is a complex number and therefore requires two real numbers to display).
Everything else can be derived, one way or the other.
Why for Z & Zpar are there 2 numbers, one with a j in front. What do those mean?
$Z$ ...
6
String trimmer line is engineered for high tensile strength, resisting breakage, but it isn't meant for 1) long term outdoor exposure and 2) long, stable lengths.
Number 2 means it will stretch over time. Not a lot, but at the lengths needed to support antennas will needed to be tightened repeatedly over the first day or two, whenever the temperature warms ...
6
What I have done once for a dipole was use a SO-239 panel receptacle with a solder cup to make my own "coax to two wire" adapter. For example, I used something like this part from Mouser. That way, I had a whole piece of coax running to the antenna that could be weatherproofed, and then an exposed section that just had regular wire soldered to the receptacle....
6
If you have a field strength meter, then you can use that. You can also use a 2nd radio, on a 2nd antenna.
But, consider reciprocity: anything you to do increase your transmit gain also increases your receive gain equally. So actually you don't need a 2nd radio, you can use someone else's.
Simply find a station that's transmitting, tune it in, and measure ...
6
Cribbing a few quotes from answers to related questions, here's a start.
From https://ham.stackexchange.com/a/195/1362:
The primary advantages of vertical antennas are that they are omnidirectional, and with an appropriate ground plane (radials) yield a low radiation angle; this reduces the number of "hops" that HF signals must make to reach their ...
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