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12
votes
6answers
24k views

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{...
11
votes
6answers
241 views

Practical limits to maintaining coherence in an HF channel

Lately I've been thinking about weak signal communication on HF. Very weak, like weaker than even WSPR could achieve. It would be nice if one could simply take any existing modulation and slow it ...
10
votes
3answers
1k views

Using ham radio bands for telemetry

If I built a machine that went far away, say a buoy, would I be allowed to use ham radio frequencies to transmit telemetry data back to its base station, if I used my call sign? This is for personal ...
10
votes
2answers
1k views

Why say “break” and “over” on walkie-talkies instead of an automatic chirp?

My understanding of why movie characters say "break" and "over" during radio conversations is based on this answer, which says that people using half-duplex devices often will want to let others on ...
10
votes
1answer
901 views

What are the general effects of a geomagnetic storm on propagation?

Per NOAA/NWS Space Weather Prediction Center, "SWPC Forecasters are anticipating G3 (Strong) Geomagnetic Storm conditions to occur January 9 and 10." What does that mean, very generally, for amateur ...
10
votes
1answer
2k views

How do I design a round or square coaxial transmission line to have a specific impedance?

Certain Hex Beam antennas use a coaxial center support post to send the RF to each antenna section, which is superior, in terms of maintenance, to a post with screw terminals connected by short ...
10
votes
2answers
742 views

Scoring quality of Morse Code “fist”?

When learning, teaching oneself, or training a class in Morse Code sending using a hand key, how can acceptable quality be scored (other than the letters being correct terms of dot-dash sequence)? ...
10
votes
1answer
560 views

Tuning a wire antenna by folding

Last weekend we tuned a quad loop for 80 m and 40 m. Instead of cropping, we folded the isolated wire using cable ties as shown in the picture below. After folding several quite large sections of the ...
10
votes
2answers
686 views

Can an amateur use cellular technologies with SDR?

Software Defined Radio is opening up many new possibilities for amateur digital modes. One family of technologies that interests me is cellular. 2G GSM voice/sms and 2.5G GPRS data has been proven to ...
10
votes
1answer
1k views

Is FM modulation still more resistant to noise when given the same bandwidth as an AM signal?

FM modulated radio is much more resistant to noise and can deliver better sound quality than AM radio. But FM signals also use more bandwidth than AM signals. In general a wider bandwidth signal is ...
10
votes
4answers
913 views

How does an antenna transform impedances along its length?

If I insert some impedance (like a loading coil, or a trap) at the base of a monopole antenna, the effect on the feedpoint impedance is easy to predict: the impedance is in series with the antenna, so ...
9
votes
1answer
232 views

Does Coax Limit the Maximum SWR a Transmitter 'Sees'?

A recent discussion about SWR led to the assertion that a length of coax cable will ultimately limit the SWR to which the transmitter will be exposed. By experimenting with an SWR calculation tool ...
9
votes
2answers
2k views

What is the largest gauge (thinnest diametre) wire I may use as an antenna?

A follow-up to Good wire for wire antenna? I'm QRT at present - thanks to fooling around with just a couple of diodes and PVC capacitors instead of a well-regulated supply. The antenna I used earlier ...
9
votes
2answers
3k views

Can every antenna be used both for transmit and receive?

Assuming I have an antenna for receiving GPS signals, can I use it also for transmit — to create a repeater in a tunnel (or some place with no reception)?
9
votes
2answers
512 views

What types of materials increase the inductance of a coil when inserted inside it?

I'm asking this because of the comments in this answer, that imply that steel (mostly iron) has similar characteristics to ferrite. Specifically, it is implied that inserting solid steel into a ...
8
votes
1answer
295 views

Why bother to have a high power base station when mobile units are generally low-power?

I want to find out why, say in a GSM/cellular system, a base station can be up to 50 watts however the mobile units can be only 100mw (for example). Surely if the base station ever uses 50w to reach ...
8
votes
3answers
1k views

Vertical antenna on HF?

What are some of the advantages and disadvantages of using a vertical antenna on high frequency? Are there any better antenna types that I should use?
8
votes
2answers
367 views

How were 1920s hams able to measure megahertz frequency?

I was reading 200 Meters and Down but one thing that was unclear to me was that hams in the 1920s were operating on the 200 meter frequency, which is around 1.5 Mhz .... or 1500 kilocycles to use the ...
8
votes
3answers
1k views

What is the proper way to ground your station's antenna?

I don't understand how you would go about giving an RF ground to your antenna. Would it not become an additional radial and affect the signal? What is the proper way to ground your station's antenna?...
8
votes
3answers
992 views

RigExpert AA-35 - Interpreting The Information

I have a new RigExpert AA-35 antenna analyzer. 1st one I have had & 1st time trying to use an analyzer on an antenna. My questions are very basic and general, but I am having difficulty in ...
8
votes
6answers
7k views

International characters in Morse Code?

Does the official International Morse code include alphabetic characters other than the 26 letters of the English alphabet? If so, what are the most common other alphabets or characters that one ...
7
votes
2answers
555 views

How to calculate if low-pass filtering is sufficient?

An answer on this question Suggested method for boosting 0.05 watt transmitter up to 10+ watts? suggests putting a filterless 19dB amplifier between the HackRF (or similar general-purpose SDR) and a ...
7
votes
3answers
1k views

"Coaxial capacitors in line” in E4E04?

I've been studying the U.S. license examination question pools. For the most part, it is reasonably obvious what the questions and answers mean even when I am not yet able to come up with the answers ...
7
votes
2answers
802 views

Store-and-forward capable satellites in operation?

The amateur satellite community used to have a small fleet of pacsat satellites (AO-16 and friends), which were capable of store-and-forward message passing: you could upload messages to the LEO ...
7
votes
2answers
851 views

What “test transmissions” are allowed above 51MHz?

The ARRL's band plan chart includes the following text in its legend: Test transmissions are authorized above 51 MHz, except for 219-220 MHz What FCC regulation is this referring to? Aren't all ...
7
votes
2answers
5k views

Is copper-clad steel a good material for wire antennas?

There are some people selling steel wire plated with copper for making wire antennas. Perhaps the most well-known brand of this product, Copperweld, makes at least two kinds of copper-clad wire, ...
7
votes
4answers
792 views

How would you get an “I'm alive” message to family in another state using amateur radio?

I'm researching options for getting an "I'm alive" message during a disaster response out to family in another state that aren't HAMs. There are likely to be a lot of possible answers that are art ...
7
votes
3answers
2k views

Impedance Matching Different Feedlines

(I promise there will be a question in this wall of text. I'm just giving context.) I've been wanting to try my hand at building my own antenna system for the last few weeks and in doing research, a ...
6
votes
5answers
1k views

Loop antenna layout and radiation?

I've got, for the near term, limited resources to put up an HF antenna at my home. One of the options I can manage without excessive cost is a loop, which would enclose the house. I need to transmit ...
6
votes
1answer
1k views

How to measuring ferrite core efficiency vs frequency

There is an unknown ferrite toroid. I would like to figure out whether it can be used in an audio transformer, or maybe in a 1:4 transformer on given HF frequency. The experiment I came up with is ...
6
votes
2answers
2k views

doppler LEO satellite

I am trying to model Doppler shift of LEO satellite, From my research so far I have come across the following formula: \begin{equation} \frac{f_r}{f_s}=\frac{1-\frac{u}{c}cos\theta}{\sqrt{1-\frac{u^2}...
6
votes
2answers
663 views

Can I transmit on 2 frequencies simultaneously?

Lets say I want to call out CQ on 145.340, 145.940, and 146.140 at the same time, am I allowed to call simultaneously on all three then monitor all three for replies?
6
votes
4answers
1k views

Can a common-mode current exist on the inside of a coax shield?

People often say common-mode current flows on the outside of the shield, while the current on the inside of the shield is always opposed by an equal but opposite current on the center conductor. This ...
6
votes
1answer
1k views

Suggested method for boosting 0.05 watt transmitter up to 10+ watts?

Long time ham operator here, new to this particular challenge - I have a software defined radio transceiver that I'd like to try and use to make contacts on HF, in particular 20-meters. Tuning ...
6
votes
2answers
578 views

Ferrite rod permeability determination

Is there a simple empiric way of determining the permeability (mhu) of a ferrite object (be it a ring, rod or any form)? I've been thinking this for a rod: if I wind coils with different number of ...
6
votes
1answer
208 views

Two antennas on different sides of a building

I live in the 4th floor of a building with one side facing south and the other north. I am able to use my 2M mobile base transceiver with a mobile magnet mount antenna placed just outside the window. ...
6
votes
2answers
204 views

For a resonant 1/2 wave dipole, does the reflections being in phase with the applied signal cause the zero reactance seen in the feedpoint impedance?

For a resonant 1/2 wave dipole when used for transmitting, does the fact that the reflected waves on the antenna are in phase with the applied signal result in zero reactance at the feed point, or ...
6
votes
3answers
4k views

What frequencies am I not allowed to use north of line A in the USA?

What frequencies can I not use if I live north of line A in the USA?
6
votes
2answers
611 views

Filtering for PSK31 demodulation

What's the ideal filter for PSK31 demodulation? That is, something that minimizes ISI, while having a narrow bandwidth to reject adjacent channels, while being reasonably matched? Most PSK modes use ...
5
votes
1answer
1k views

How to calculate or estimate surface area of required ground plane for a mobile antenna?

I don't have conditions for installation of a fixed 2 m/70 cm antenna at home, so I'm thinking of using a mobile magnetically mounted antenna on a conductive plate instead. So my question is: What ...
5
votes
2answers
552 views

What are the implications of pulling LMR400 through EMT conduit with several 90 degree conduit body fittings

In order to route my antenna feedlines out of my house, I have to pass through a wall, into the garage and through another wall. All of this involves four 90 degree turns. I am wondering what the ...
5
votes
2answers
3k views

What does the term 3 kHz deviation relate to in the FM world?

I am confused when someone says "your audio levels are set right and has good deviation at 3 kHz" when talking about wide-band 25 kHz FM signals. First confusion: Why are audio levels measured in ...
5
votes
3answers
2k views

Do I need an 9:1 UNUN for a half-wave end fed?

I was using a 10-ish meter wire as my HF rx antenna. That's all I can install in my QTH. However I realised that my ~10 m wire can be turned into an end fed dipole for 20 meter band. But there is a ...
5
votes
2answers
390 views

Will measuring SWR through a feed line mislead me?

I know that except in the perfectly matched case, transmission lines transform impedances along their length; if I measure the impedance at the other end of my feed line, or even with a short jumper ...
5
votes
3answers
401 views

Is compression “encryption” under FCC regs?

I read this question about digital signatures and FCC prohibitions on "obscuring" messages in amateur transmissions, and it cause me to think of something: the difference between encryption and ...
5
votes
3answers
3k views

Are there any good, consistently receivable signals in the US for HF receiver testing?

Having built my first HF receiver, I'd like to know if I'll need to search around for transmissions that happen to be occurring, or if there are reasonably strong signals I can count on being present ...
5
votes
2answers
205 views

Amateur Radio for a Nomad [duplicate]

I got my amateur license back in the early '80s, primarily for putting it on my resume for an electronics job. Never used it outside of developing my Morse code speed, and once I got my Advanced Class ...
5
votes
3answers
282 views

How far the near-field reactive region extends from the transmitting antenna?

I am doing research on near-field harvesting. In my situation the transmitting antenna is a dipole antenna operating at 2.45 GHz and its length is 14 cm long. I am trying to find how far near-field ...
5
votes
3answers
16k views

How to improve reception with a headphone-cord antenna?

I have a headphone and a cellphone that has built-in FM radio, and the headphone acts as the radio antenna. but my problem is the radio stations I listen to are very far from my place, good thing ...
5
votes
1answer
242 views

What do hams mean when they refer to “higher bands”

For example, the Ultimate3S kit info page says: Produces 250mW RF output on 30m (lower output on higher bands) Would bands "higher" than 30m mean: bands with higher frequency e.g. 20m/17m/15m? ...

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