30
votes
How does the signal know where the cable ends and the antenna starts?
An excellent question! Without diving too deep into the theory, let's start with a few basic terms.
The "signal" that an antenna is receiving or transmitting is called an electromagnetic wave. This ...
- 18.5k
17
votes
Accepted
How did hams manage to tune their antennas before VSWR meters?
Mostly, they used RF Ammeters in series with the antenna. The first ones were hot-wire ammeters which were completely mechanical devices. One end of a thin nichrome wire (or other wire of sufficiently ...
- 7,864
13
votes
Accepted
What is an antenna tuner? Why bother with resonant antennas in the first place?
And if it is possible to use a tuner to make a transmitter feed power into a coat hanger then why do we worry about making resonant antennas in the first place?
In a word - efficiency.
Consider that ...
- 18.5k
11
votes
Accepted
Attic Dipole - Impact of Ethernet over CAT5/6
Having Ethernet and your antenna co-located is not an ideal situation. But then most amateur antenna situations involve compromises. The general idea of the following recommendations is to take as ...
- 18.5k
10
votes
What is an antenna tuner? Why bother with resonant antennas in the first place?
My understanding is that they can neutralize the imbalance between inductive and capacitive reactance so the transceiver can couple the most power out (or in) even though an antenna might not be ...
- 51.6k
9
votes
Two antennas on different sides of a building
As one solution, you can combine the antennas with a power divider. See How to combine two 50 Ω antennas such that they appear as one 50 Ω load?
This makes your pair of antennas into a phased array. ...
- 51.6k
9
votes
How does the signal know where the cable ends and the antenna starts?
Antennas are often resonant. Their physical dimensions are adjusted so standing waves develop at a particular frequency, like a bell rings at a particular tone.
Feedlines are not usually resonant. ...
- 51.6k
8
votes
Accepted
What exactly is impedance?
As resistance is to DC circuit analysis, impedance is to AC (or RF) circuit analysis. Now let's take a closer look at what that means, from several angles. I'm going to not include any of the math, ...
- 24.4k
8
votes
Share wide band antenna with multiple transceivers
The basic concept is to use RF bandpass filters for each frequency range. This is frequently done by hams for VHF and UHF applications. The common term for a grouping of these filters is diplexer, ...
- 18.5k
8
votes
Accepted
VHF 50 Ω Antenna Over 75 Ω TV Coax
I will need a UHF/VHF diplexer on either end to suitably merge/split the signals from each antenna
Yes, this is correct.
A tangent: If you wanted to save some money by using mass-market parts, ...
- 24.4k
8
votes
Antenna Basics - Open/Short Circuit & Half-Duplex/Full-Duplex
My Understanding 1 - Antenna is just a piece of a metal conductor. "Antenna is an OPEN CIRCUIT, and not a SHORT CIRCUIT". Is my understanding correct that the Antenna is an open circuit and not a ...
- 24.4k
7
votes
Accepted
Share wide band antenna with multiple transceivers
A wideband antenna is not what you're looking for – you really don't care about anything between 900 and 1500 MHz, or between 1600 and 2400 MHz.
Wideband antennas are inherently hard to make, and ...
- 15.5k
6
votes
Accepted
What dictates ladder line max frequency?
1% here is just an overly precise way of saying "really small compared to the wavelength". The problem is that when the conductors are close together, their electrical and magnetic fields cancel; as ...
- 950
6
votes
What would be the easiest and most cost effective ground system for a startup UHF/VHF station
That statement is wrong on several levels.
The antenna is DC grounded so no lighting arrestor is needed.
A lightning arrester is needed, even if the antenna is DC ground. The arrester's job is to ...
- 51.6k
6
votes
Accepted
Does Coax Limit the Maximum SWR a Transmitter 'Sees'?
The source of the SWR limit on the transmitter end is the losses in the feedline. In general, the higher the matched line loss, the lower the maximum SWR that will be present at the transmitter end. ...
- 18.5k
6
votes
Using 75 Ω instead of 50 Ω coax feed
Sure, you can use 75 ohm coax instead of 50 ohm. In some cases (such as feeding a dipole), 75 ohm coax may be a better match to the load than 50 ohm. In other cases (such as feeding a vertical), it ...
- 51.6k
6
votes
How to match low impedance of NVIS Dipole?
Many baluns will work just fine in either direction, though there isn't just one kind of "4:1 balun".
This kind is wound on two cores, and works as a common-mode choke:
simulate this circuit – ...
- 51.6k
6
votes
Accepted
Why do we talk about common mode current from an antenna, but not from a transmitter?
The short answer is that it can't.
A shielded transmitter, connected to an ideal piece of coax, does not generate common mode currents.
The inner and the outer of the cable look connected (and for ...
- 10.5k
6
votes
Accepted
If receive performance doesn't depend on SWR, then how can I tune my manual HF antenna tuner by band noise?
Receive power does vary with SWR; but for reception quality we're interested in signal-to-noise ratio. As long as you're getting enough receive power that the noise received from the antenna is ...
- 11.7k
6
votes
Accepted
Where can one get a "listed" SO-239 lightning arrester?
The NEC defines "listed" as:
Listed. Equipment, materials, or services included in a list published by an organization that is acceptable to the authority having jurisdiction and concerned ...
- 51.6k
6
votes
Is there a proper way to connect multiple radios via coaxial cable so they can be tested without going over the air?
My company built a system like this, to allow real radios to be used in a flight simulator. It had fixed and digital variable attenuators, so it could simulate exactly the effect on communication of (...
- 10.5k
5
votes
Connect two receiving antennas to the same radio
Rather than two receivers, maybe a switch that selects between antennas could be a cheaper option?
- 51
5
votes
Accepted
Antenna for a cramped attic
Your attic is pretty low but not quite low enough to be a problem for installing a vertically polarized 144 MHz antenna. There are two common antenna designs that would suit your case:
Ground plane ...
- 24.4k
5
votes
Which is better in my situation: Balanced dipole, or as much wire as possible in the air?
I think in practice you'll find it doesn't make much difference as long as there's a common mode choke or "current balun1" at the feedpoint. In fact, if you get it off-center in just the right place ...
- 51.6k
5
votes
Accepted
Use a TV coax and balun for a poor-man's OCFD?
OK, let me try to answer this, but this answer may also be qualified as unqualified.
If you have a 50 Ohm receiver, and connect a perfectly (Z=R) 75 ohm antenna system, then your VSWR would be 1.5, ...
- 2,137
5
votes
TV strong station has no signal when coax is connected
I'm not at all sure about the exact situation, but I predict that you will find that the cable between the distribution amplifier and this particular TV set is damaged.
In general, when you omit the ...
- 24.4k
5
votes
TV strong station has no signal when coax is connected
You may be overloading the front end of the "faulty" TV, creating adjacent channel interference or some other kind of distortion. The loss you introduce by only loosely coupling the receiver may be ...
- 542
5
votes
Accepted
Why do we distinguish between balanced and unbalanced?
There are balanced feedlines (twin-lead), and balanced antennas (dipoles), and similar things all described as "balanced". In this sense, balance means the two halves of the thing have equal impedance ...
- 51.6k
5
votes
Accepted
Electrical Earth
Congratulations on getting permission to install an antenna. There are many hams that rent that would love to be in your position.
As Phil correctly asks, why do you need an earth connection? Here ...
- 18.5k
5
votes
Are Elevated Radials Necessary for Elevated, 5/8-wave "Ground-Plane" Antennas?
Elevated radials are required in order to increase/optimise the radiation possible from any such elevated "GP" monopole or whip antenna.
If the elevated radials are symmetrically arranged and lie in ...
- 2,922
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