28 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 ...
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  • 18.1k
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 ...
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  • 7,495
12 votes
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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 ...
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  • 18.1k
11 votes
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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 ...
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  • 18.1k
9 votes
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Connect two receiving antennas to the same radio

Yes, you can do this. There are two issues: connecting it to the radio, and the effect on reception. The latter might be problematic enough that using two receivers is actually a better option. ...
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  • 22.9k
9 votes
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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, ...
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  • 22.9k
9 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 ...
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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. ...
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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. ...
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8 votes
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Share antenna between receiver and transceiver?

Back in the olden days, before transceivers took over, the transmitter and receiver were separate units, and the receiver had to be protected from the transmitter. The solution then, as now, is an ...
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  • 940
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, ...
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  • 18.1k
8 votes
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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, ...
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  • 22.9k
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 ...
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  • 22.9k
7 votes
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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 ...
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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 ...
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6 votes
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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 ...
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  • 940
6 votes
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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. ...
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  • 18.1k
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 – ...
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6 votes
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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 ...
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  • 7,817
6 votes
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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 ...
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5 votes
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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 ...
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  • 22.9k
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?
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  • 51
5 votes
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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, ...
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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 ...
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  • 22.9k
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 ...
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  • 432
5 votes
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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 ...
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5 votes
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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 ...
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  • 18.1k
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 ...
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  • 2,872
5 votes

Is it practical to use a slug-tuned variable inductor in an antenna matcher (aka "antenna tuner")?

...variable caps are ... no longer manufactured in the old "interleaved plates, air spaced" form, and the tiny plastic dielectric ones that are still available can't take much voltage (and are ...
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  • 7,685
5 votes

Is it practical to use a slug-tuned variable inductor in an antenna matcher (aka "antenna tuner")?

Fair-Rite makes ferrite rods which would be suitable for HF applications. It should be possible to create or repurpose a screw-operated mechanism to move the rod into and out of a cylindrical coil. ...
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  • 7,685

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