Questions tagged [physics]
18 questions
35
votes
4 answers
Effect of different metals for antenna elements
Other than rust, corrosion, and other reactions with air that would make the use of a metal unfavorable, how do different metals affect the performance?
Let's give Yagi an example:
Let's say I use 4 different metals for the directors
, reflector,…
Skyler 440
- 7,550
- 7
- 48
- 88
14
votes
3 answers
Why do concurrent FM signals not mix together?
Generally when I hear two conflicting signals they do not mix together, but rather one dominates then the other. This phenomenon can be observed on FM broadcast station when driving out of range from one station into the range of another. During the…
Dan
- 1,754
- 3
- 16
- 36
13
votes
6 answers
Calculating Antenna Length on the FCC Exam vs. in Reality
I have seen the following formulas for calculating half and quarter wavelength antennas:
Formula for $\frac{1}{2}$-wavelength antenna (in free space):
\begin{equation}
\mathrm{Length~(feet)} = \frac{492}{f_{MHz}}
\end{equation}
Or is…
Dan
- 1,754
- 3
- 16
- 36
10
votes
1 answer
Why is the speed of light rounded to 286 Mm in calculations involving frequencies below 30MHz?
I understand that in order to make the maths simpler, frequency ($f$) is expressed in megahertz (MHz) and the velocity of propagation in free space ($c$) for frequencies above 30 MHz is expressed as and rounded to 300 megameters/second (Mm/s). The…
Dan
- 1,754
- 3
- 16
- 36
9
votes
1 answer
What's this "crossover" toroid winding called and what is its purpose?
I am winding an EFHW matchbox and many of the designs I've found (including the diagram provided with the kit I bought) include a sort of "twist" when winding them:
(Image taken from this post and similar can be seen here and here and here.)
My…
natevw - AF7TB
- 7,219
- 2
- 23
- 58
6
votes
4 answers
How far do radio waves travel?
I have read that radio waves just keep traveling but the signal gets weaker because of how the wave spreads. OK, I get this, but what confuses me is that satellites emit signals from space that our phones and GPS modules pick up regardless of…
sirshakir
- 61
- 1
- 4
5
votes
1 answer
Does a low amount of atmospheric noise reliably indicate a lack of propagation?
We know that the primary source of interference at HF is atmospheric noise. Thus, all else being equal, the strength of heard noise is a fair indicator of RX system performance (such as in adjusting an antenna tuner for maximum noise while…
Kevin Reid AG6YO
- 24,195
- 7
- 48
- 101
5
votes
1 answer
Why does the ionosphere reflect some frequencies and not others?
Why is it that some frequencies transmitted at some known direction are reflected back to earth, yet others transmitted at the same direction, power and polarity pass through into space with no reflection?
BenAlabaster
- 263
- 1
- 3
- 10
5
votes
3 answers
When a dipole antenna is fed via coax, what powers the ground-side of the dipole?
I've been trying to understand how dipole antennas work, and I believe I understand the basics when they're fed via a balanced feed line. In fact, every dipole physics YouTube video and web page I can find explains how they work with a balanced feed…
Asker2093022
- 51
- 1
5
votes
3 answers
Energy vs power in transmitters
Higher frequencies have a higher energy, simply stated by Planck's constant. When I am transmitting 100 watts on 500 kHz, and transmitting 100 watts on 5 GHz, is there an energy difference in the wave?
Skyler 440
- 7,550
- 7
- 48
- 88
4
votes
2 answers
Antenna Design for specific directions
I am designing an antenna for a device and was hoping to gain some insight in the correct design direction for the antenna.
The Idea: A receiver & antenna that fits in a 6" diameter by 3" deep cylinder.
Known: Frequencies range from 162.400 to…
Josh Cox
- 241
- 2
- 7
4
votes
6 answers
Do changing sidebands have their own sidebands?
An amplitude changing carrier wave causes sidebands to appear next to it when viewed in the frequency domain. But if those sidebands themselves also change with time (because the modulating signal is not constant), shouldn't those themselves behave…
JanKanis
- 405
- 2
- 11
3
votes
2 answers
What do radials really do for a vertical antenna?
I have read everything I can find about monopole (vertical antenna) radials, and also watched many YouTube videos that touch on the topic. I see conflicting information about what they do, where a particular article or video tends to fall into one…
User5754448
- 438
- 3
- 10
3
votes
2 answers
Using approximate signal strength at a distance to estimate reception strength of a radio station
I have some historical data on radio stations, but unfortunately, the dataset only has these variables:
the power of the transmitter, in watts
the coordinates of the radio tower
Unfortunately, I don't have any data on the characteristics of the…
Michael A
- 185
- 7
2
votes
1 answer
How to calculate phase constant for a coaxial cable?
inner and outer conductor diameters of 1 mm and 5 mm respectively, is filled
with a non-magnetic PTFE dielectric having εr = 2.55. frequency = 1 GHz.
Praskand
- 21
- 4