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33
votes
7answers
31k views

Using a balun with a resonant dipole

I am preparing to construct a resonant, center-fed, horizontal dipole. I have seen people use baluns at the feedpoint of the antenna, and some people swear by them, but I have never gotten a ...
25
votes
4answers
3k views

What is a link budget, and how do I make one?

Like the question title says, what is a radio link budget? For what is it useful, and how do I make one?
18
votes
5answers
6k views

How can I protect equipment against a lightning strike?

A lot of aerials (almost by design) would be rather good attractors of lightning. What steps do I need to take in the field to keep my gear safe against a lightning strike? I'm presuming the obvious ...
16
votes
1answer
7k views

How to detect common-mode currents or “RF in the shack”?

Let's say we already know that if your antenna is not suitably constructed or connected then you can get current on the shield of your coaxial feed line which will flow through the shielding/grounding ...
23
votes
4answers
8k views

What is the relationship between SWR and receive performance?

If an antenna analyzer shows 1:1, does that mean it's an ideal receiver as well? And what about the converse, will a well performing receive antenna show a 1:1 SWR? I've wondered this for a while, ...
6
votes
1answer
2k views

What does the Friis transmission equation represent and how is it derived?

I've read that the Friis transmission equation can be applied to calculate path loss in ideal space. However, I'd like to better understand the factors that define it and what it represents, and why ...
7
votes
3answers
1k views

What is the actual loss in a feed line with high SWR?

It is well-known that it is unwise to have standing waves on the feed line between one's antenna and one's transmitter, unless it is a very low-loss type (e.g. ladder line), because the transmitted ...
14
votes
2answers
4k views

Is free space path loss dependent on frequency?

Free space path loss is calculated as: $$ \left( 4 \pi df \over c \right)^2 $$ The presence of $f$ in this equation means that losses increase with increasing frequency, so there's an inherent ...
9
votes
6answers
26k views

What is a Gamma match in the context of the driven element of a Yagi antenna?

How (and why) does a Gamma match work, when used on the driven element of a Yagi antenna? As shown here: (source: http://www.iw5edi.com/ham-radio/?2-element-yagi-for-10-meters-band,49) The article ...
6
votes
5answers
8k views

Wifi passive repeater (antenna to antenna connection)

I have two buildings. Building A has wifi (2.4GHz) infrastructure, Building B doesn't. One side of building B can pick up the wifi signal from building A. I want to passively extend the signal through ...
8
votes
1answer
482 views

Why must twin-lead conductor spacing be small to avoid radiation?

A transmission line does not radiate when the spacing between the two wires is very small when compared to the wavelength, but the transmission line begins to radiate when the spacing between the ...
32
votes
2answers
43k views

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 ...
12
votes
2answers
3k views

How does one read a ferrite datasheet?

Datasheets for ferrite materials are usually filled with incomprehensible graphs with greek letters. For example, Fair-Rite's 73 material: What's a $\mu'_s$ and a $\mu''_s$? What do these numbers ...
18
votes
1answer
3k views

How does moving a feedpoint off-center in a dipole affect the resonant frequency and resistive load?

Specifically, I understand that OCF dipoles change the SWR, but does it do this via reactance or resistance, or both?
7
votes
1answer
270 views

How can I calculate the effects of an LNA, antenna gain, etc. on noise performance?

There are many things I might do enable my station to hear weaker signals that would otherwise be buried in noise. Just a few possibilities: Install a nice LNA Increase antenna gain Reduce feedline ...
9
votes
2answers
3k views

What causes ground losses in a HF antenna system?

This applies to mobile and fixed installations. What are ground losses, and how can they be minimized? It seems to involve proximity to the ground, but what is the mechanism that causes loss? Does ...
18
votes
3answers
7k views

Why do I need to tune an antenna?

I've heard that when an antenna is set up, it needs to be tuned to reduce reflective power back into the radio. Is this always necessary, and if so how would I go about tuning it?
9
votes
2answers
3k views

Does a balun need to be made with coax?

The short version My understanding is that one of the main reasons to use a balun with coax is to prevent the outside of the shield from becoming part of your antenna system. If that's the case, won'...
15
votes
3answers
8k views

What is the peak voltage at the tips of a dipole antenna?

What is the peak voltage present at the very end tips of half-wave dipole antenna in free space, and how might this peak voltage relate to transmitter type, transmitter power, RF frequency vs. antenna ...
14
votes
4answers
4k views

What does the addition of a shield to a small loop accomplish?

A small shielded loop is an antenna with a loop of wire, smaller than 1/10th of a wavelength in diameter, also surrounded by a shield. Often, they are constructed from coax. There is a gap in the ...
13
votes
4answers
2k views

Handheld Dipoles

A problem with handheld devices is poor grounding. I read from someone else's question that dipoles don't need a ground. Why don't they make little handhelds with two antennas instead of one, or a ...
12
votes
1answer
5k views

How is the IF for a superhet selected?

There seem to be almost as many choices for the intermediate frequency (IF) as there are superheterodyne receivers and transceivers out there. The ones I have come across are 455 kHz, 452 kHz, 1.6 MHz ...
22
votes
13answers
15k views

How can one convert from Lat/Long to Grid Square?

I see that many contests, awards, and other items, use Grid Squares as a means to identify where one is. How can one figure out what one's grid square is, given lat/long?
17
votes
8answers
24k views

Should I chose a vertical or a horizontal HF antenna?

If I only have the budget for one HF antenna, should I chose a vertical or a horizontal antenna?
5
votes
2answers
10k views

What is the aperture efficiency of an antenna?

For reflector antennas, I see a number called aperture efficiency. What does it mean?
4
votes
1answer
3k views

Can I improve reception with a more sensitive antenna / preamplifier / better feedline?

I am trying to receive a station, but I can only barely hear it through the noise. Will doing anything that increases the signal from my antenna help, such as: using a more sensitive antenna adding a ...
10
votes
2answers
3k views

Is there an optimum Antenna Feedline Length for mobile installations?

I bought a ham radio antenna mounting kit for my car that includes ~16' of coax. One end has a trunk lip NMO antenna mount, the other end has a re-solderable PL-259. I asked the sales person if it is ...
9
votes
5answers
2k views

What is an antenna tuner? Why bother with resonant antennas in the first place?

There are a lot of antenna tuners on the market that have various capabilities in terms of how they can allow random radiators to work with our transceivers. What exactly do they do? My ...
10
votes
2answers
3k views

How to reduce RF in the shack when using vertical HF antenna (no radials)

What can be done to reduce RF in the shack (riding back along the coax) when using a vertical 10M antenna (without radials due to space constraints)? The antenna is an IMax-2000 at 32' elevation (20' ...
6
votes
2answers
4k views

How does a folded balun work?

A folded balun is made from two quarter-wavelength, parallel conductors. A picture is worth a thousand words: Here are some pictures of an actual construction from Gisela & Joe Noci on diydrones....
0
votes
5answers
617 views

How to test Line-of-Sight radio at 5GHz at range 25Km or less?

I'm thinking of buying PowerBeam ac. It's a wireless device to connect two computer networks that are about 25 kilometers apart. But it depended on line-of-sight they say. So is there a cheap way to ...
44
votes
5answers
4k views

How do I report an emergency using ham radio?

If I'm in the wilderness with ham equipment but no phone signal, how would I report an emergency? Is there a particular frequency I would use, a particular protocol or suchlike?
58
votes
16answers
7k views

Why is Morse Code still in use?

Why do people still use Morse Code? What are its advantages over newer Voice or Data communication modes?
23
votes
2answers
56k views

Which HF bands are best during the day and which are better at night?

I am rather new to HF operation and have recently purchased my first HF rig. I have read about bands being "open" and "closed" based on various conditions including time of day. As I have been ...
28
votes
6answers
28k views

Can I reduce RFI/noise at the antenna?

Recently, the power was out in my neighborhood for an extended period of time. I fired up my HF rig using battery power and was amazed at the lack of noise across the HF bands. When the power grid ...
23
votes
1answer
5k views

Is there a way to listen to live HF communications using the internet?

I'd like to be able to show people what a QSO sounds like without having my transceiver available. Is there a way I can listen to amateurs talking in real-time (or using Morse code) on the HF bands ...
17
votes
1answer
2k views

How does an SWR meter really work?

For ham radio operators, the SWR meter is a ubiquitous piece of equipment. There are dozens of standalone models on the market and most modern transceivers and antenna tuners have this functionality ...
15
votes
7answers
8k views

What is the cause of this unusually long period of terrible HF band conditions?

I've been a ham for over 40 years, but never has there been such a prolonged period of such poor propagation. This has been especially the case over the past six months or so, and they continue to ...
14
votes
2answers
15k views

How do I make a RF propagation map for a repeater?

Let's say I have a repeater. Knowing all of the information about antenna height and gain in each direction, as well as power, how can I create a map similar to this map I saw of a repeater in ...
9
votes
5answers
25k views

Is there a simple, DIY, antenna suitable for HF receive only?

So the antenna I'm thinking about doing might be* way too large for my property. Is there another simple, cheap, DIY antenna I could try on my receiver? I know I'm going to want to hook it up and ...
6
votes
6answers
4k views

Why is antenna aperture a function of wavelength?

Wikipedia says: It can be shown that the aperture of a lossless isotropic antenna, which by definition has unity gain, is: $$ A_{\mathit{eff}} = \frac {\lambda^2}{4 \pi}$$ We also see this ...
12
votes
2answers
544 views

What are the (US) rules about unlicensed low-power transmissions?

It is common wisdom that transmissions of any sort (perhaps unidentified, encrypted, in the FM broadcast band, etc.) are permitted if they are sufficiently low power. Where are the FCC regulations ...
20
votes
2answers
3k views

How do Handheld antennas work without ground?

Dipoles have the 2 wires going to each leg of the antenna completing the circuit. Big AM towers use the second wire as a ground plane But how to handheld rigs transmit and recieve with just one wire ...
13
votes
4answers
3k views

How does a folded dipole work?

A folded dipole is like an ordinary dipole, but with the ends extended and folded back, until they meet. Although it looks like a loop, I'm told it behaves similarly to a dipole. How does this thing ...
9
votes
3answers
2k views

Why does “high SWR” damage transmitters, instead of “impedance mismatch”?

In the audio realm, where cable lengths are insignificant compared to the signal wavelength, I would understand an impedance mismatch damaging an amplifier as follows: Take an audio amplifier that ...
7
votes
5answers
14k views

On which of these frequencies can I transmit without a license?

I have a Baofeng UV-5RA, and I need to know what frequencies on the "channel mode" (what the lady says when you press the "V/M" button on the radio) don't require a ham license. I am in Philadelphia, ...
6
votes
2answers
732 views

Dipole Antenna Current Distribution at any Time Instant

Above is a connection of half-wave dipole antenna. The book states that the current profile is same at any given time instant on both the radiators, each of quarter wavelength length. Now my ...
6
votes
1answer
490 views

Can tropospheric scatter effect be used by 802.11 links?

What are the lowest EIRP and antenna gain requirements that make possible use of troposcatter at the distances of 100 to 150 kilometers assuming conventional WiFi (6 Mbit/s data rate with OFDM/BPSK ...
5
votes
2answers
162 views

What are the requirements of the address given for a US licence?

I'm going to be visiting the US soon (I live in the UK), and since I've heard it's very easy to get a US amateur radio licence and it's something that could be beneficial, I thought, why not? I see ...
5
votes
2answers
852 views

Adapting an antenna coupler

I've built AA5TB's antenna coupler: I left out C2 because I didn't understand why I would need it - why would I? It works fine without. Unfortunately, when I started tuning for a 5W 20m transceiver, ...

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