New answers tagged

1

The pictures look a lot like post-socialist country big city residental area. It may as well not be, YMMV. The Yagi looks like an abandoned old analog TV antenna. Most ham installations use vertical polarization in UHF and this one looks pretty much horizontal. The cable between the buildings is not an antenna at any rate. This is a communincations cabe of ...


1

Two classic and highly effective fractal antennas are the log periodic antenna and the many variations (planar, conical, cylindrical, etc.) of the log spiral antenna. The main characteristic of both of these antenna families is extremely broad bandwidth, which can be attributed to the fractal self similarity at multiple scales. Most other fractal antennas ...


3

The cable strung between the two buildings has two parts, a strong wire probably made of stainless steel or similar, then you can see some form of cable, as Tommexus said it's probably fibre or telephone cable, or maybe coax, hanging from the wire with lots of evenly spaced cable supports. The steel wire is used to provide a strong support between the ...


10

It looks like a fibre or telephone cable strung between buildings. It sets a good precedent for setting up your antenna though - if you can get access to the other rooftop at night, and a catapault or fishing rod. Make it fairly official-looking, with some large bolts and a labels with bar codes and lots of numbers.


0

I see knots being used for desktop lamps, after the wire enters the lamp, with stranded-wire. #1.) The more sharp the bend, the more you stress the wire and prepare it for breakage. As long as it's just one time, and no repeats, no bending and unbending, you should be fine. #2.) If you want to be safer, slip a little heat-shrink on first, tie the knot in the ...


-2

To complement Brian K1LI and his Fractal antenna, I thought I would give you another design which is more space-filling, and may yield better results for you. (see graphic at bottom). Their application is antennas for cars. You can see how their space-filling-fractals (the squares/diamonds) are electrically much longer, and therefore fit into smaller spaces (...


2

The formula in the question is taken from page 44 of a book called The Theory of Electromagnetic Wave Propagation by CH Papas. The text says that the formula describes the radiation pattern for a center driven wire antenna. Radiation patterns normally show a graph of relative intensity which isn't the same thing as efficiency. For transmitting, the ...


1

It isn't. Gain in the broadside (θ=0) direction isn't "efficiency". A half-wave dipole doesn't maximize that parameter, even among dipoles; it increases until roughly 1.25 wavelengths, then decreases, then generally increases again, in an oscillating way. There are nice things about half-wave dipoles (a convenient, easily-matched impedance and a ...


3

Nothing. A G2 storm is a common event; according to NOAA data it happens an average of 600 times per 11-year sunspot cycle. It won't hurt your radio, but it might make HF communication a little more difficult. In fact, no category of storm is expected to hurt your radio in particular, although G4 and G5 storms can cause major radio blackouts, not to mention ...


2

I'm not sure if this is the correct answer or not: At varying electric lengths, at any moment in time, one part of the antenna is producing the wanted field(s) and another part might be producing a canceling field(s). With a half wavelength, all you get is the wanted field and no canceling field. The canceling field (I call it that because it works against ...


0

In addition, the $S_{11}$ parameters of the both TX/RX antennas should be typically less than -15 dB for a better measurement in the frequency interval that you sweep. Then, you will have a better observation about how much the received signal power decreases as the operating frequency increases. Otherwise, your measurements will be affected by the ...


0

I would refrain from knotting antenna wire meant for a long-term installation. Knots in rope weaken the rope by 20% to 60% (source). I don't know of any research on knots in antenna wire, whether kevlar-reinforced or "knot" (sorry, couldn't resist the pun), but it seems reasonable to expect that tying a knot in antenna wire weakens it. You might ...


0

Generally, stranded wire used for antennas is fine to knot. However, thin solid wire that is knotted or kinked can develop a weak spot that can break under tension. You would not, for example, want to knot load bearing cable used as guy wire, even at the tips. Instead, you can use a wire thimble (selected as appropriate for wire diameter and alloy). ...


5

It should be fine. Stranded wire will take that kind of bending easily. If you were to tie and untie the knot many times you might eventually weaken and break some of the strands, but if it's tied once and left in position, it won't create so much stress. If you're worried, of course, there are alternatives — you could slide a small plastic bead onto the ...


3

It's all to do with mutuals, and you can't wish them away - even for 'ideal' antennas. Consider a simple case, two elements a reasonable distance apart. Give them excitation amplitudes +1 and -1. From your assumptions, the radiated power is 2, irrespective of their locations. The power is no longer radiated isotropically, there is some array pattern, but ...


1

If you need calibrated output then you probably won't be able to do it. That's why they are so expensive. It's like a \$20 SDR dongle versus a \$2,000 spectrum analyzer, with the most notable difference being that the spectrum analyzer is calibrated and tells you how much power is at each frequency. If you don't need calibrated output and just want to get a ...


1

You don't need a spectrum analyzer for such low frequencies, more like a very accurate voltmeter that you can record over long periods. As the antenna, you will need something huge, I recommend the power grid. Filter everything out above 59 Hz and see what you are left with. Might have to do some different filtering to try and find Schumann resonances. Make ...


1

Consider a simpler problem: Stack two dipoles at 0.01 wavelength separation. Connect the endpoints and feed power to one of the dipoles and short the feed-point of the other. What you then have is a folded dipole. Impedance 300 ohms. Then remove the short and feed both feed-points. The impedance of each one will be 150 ohms. They are series connected on a ...


2

Years ago I used a simulation like this one for the same purpose. The antenna you refer to is a tuned antenna, so it is not wideband. The antenna that I made is a wideband, really flat response (conversion from field strength to output voltage is frequency-independent). Conclusion: the difficulty is the design of the low-noise amplifier. Depending on what ...


6

I know that the gap between the wall and the antenna mast that would be created using the wall mounts you mention is not for lightning damage prevention purposes. If a bolt of lightning can travel hundreds of meters from a cloud to the ground, another half-meter of air between a mast and a building won't be much of a deterrent, especially when the bolt ...


1

$Z_{fs}$ (space) is resonant at all frequencies due to $\varepsilon_0$ (farads/meter) and $\mu_0$ (henries/meter). Thus it will have no reflection. Free space is lossless and the signal only gets weaker because the energy spreads out over a larger and larger sphere. The /meter part causes space to develop a resonant velocity (speed of light). But all ...


1

The likelihood of an antenna interfering with another antenna diminishes as their respective frequencies get further apart. Typically, if they are on different non-harmonic bands, it won't be an issue, and if they are harmonic, the higher frequency antenna might cause interference on the lower frequency antenna, not the reverse. Dishes are probably above ...


2

here is a great answer to a similar question, "how do I calculate the bandwidth of an antenna". Assuming the antenna is in free space, you only need to know the length and diameter of the wire used to construct the dipole. The math is hairy but I wrote a program to do the calculations. Here is the SWR (assuming a 50 ohm source) and feedpoint ...


2

Of course there can be interference but I would guesstimate the probability at about 30%, so that 70% chance that there will be no interference - basically have to try to find out. If there is interference you can move the antenna a little bit and that might even fix it or install a filter that might fix it, or use less power on certain frequencies. I would ...


3

As already mentioned by @Andrew's answer; as a general rule: the higher the better. Another very important rule is: any antenna is better than no antenna. So a hex-beam on top of a 24 floor building is better than none ! Lightning was mentioned, which is something you need to give serious thought. Try to find out who is the facilities engineer, and have a ...


3

The general rule of thumb is that for antennas the higher the better. The height above ground in this case is an absolute advantage. Your ability to put an antenna on top of a very high building is enviable to say the least. Height above ground affects the angle of radiation, which is the vertical angle compared to ground of maximum radiation for an antenna. ...


5

A choked end-fed antenna is (or acts just like) a highly off-center fed dipole, where the length of any stub between the common mode choke and the unun/transformer/feedpoint sets the off center ratio and thus the impedance at the antenna feed point. Your current 1 foot stub is less than 1% of the antenna, whereas a few percent is usually recommended, that ...


3

This question is more complicated than it seems. If the antenna in question is only being used at one frequency, and at that frequency the SWR is already low, then in terms of obtaining a good match to transmission line and radio there is no advantage in using an antenna tuner. In fact the tuner would introduce a very small amount of insertion loss, and this ...


4

This is one of those areas where "words matter" The typical "tuner" may actually accomplish one or more of several functions: matches impedance between two "things", for ex a radio to a feed line or a feed line to an antenna; tranforms a balanced line to an unbalanced line, or vice versa introduces complimentary complex ...


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