Kv4p HT – A homebrew 1W radio (VHF or UHF) that plugs into an Android phone

181 pointsposted 14 days ago
by krupan
(kv4p.com)

85 Comments

thenthenthen

11 days ago

There are many cheap shanzhai android phones with walkie talkie ptt functions (around 400MHz iirc) for about a decade. We tried to convince manufactures to open up the software stack to no avail, while being bullied at local hackerspaces (“this is illegal!”). I am licensed.

lxgr

11 days ago

It's mindblowing to me how modern cell phones have worse broadcast and P2P capabilities than what we had decades ago (when feature phones often featured FM receivers and could share photos via Bluetooth across manufacturers and OSes).

Airdrop was the closest thing we had, and even that has been intentionally nerfed for non-contact senders.

It's absurd that modern phones can talk to satellites hundreds of kilometers above, but not to other phones a few meters away in the same room, airplane cabin, train car etc.

wolvoleo

11 days ago

P2P capabilities aren't what the providers and governments want.

The providers don't want it because they can't charge you for it. The governments don't like to see people communicate outside of their control. See how Apple caved to China making AirDrop no longer public and has followed suit in the rest of the world because other governments fear this capability too.

euroderf

11 days ago

So, do phones in the "libre" genre have these features ?

wolvoleo

11 days ago

No, but I think this is more because it takes them so much effort replicating the basic functionality and compatibility with e.g. android apps that they just don't have the energy for it.

Also, the hardware for good peer to peer communication is just not present in regular phones. Some libre hardware projects do have optional addons for LoRa, such as the PinePhone: https://pine64.org/documentation/Phone_Accessories/LoRa/

dTal

10 days ago

That's not at all true, phones have both WiFi and Bluetooth, both of which are perfectly capable of communicating peer to peer. Bluetooth file transfer worked 20 years ago. Wifi is physically capable but the software usually requires one phone to be the Daddy and host a hotspot, because god forbid an IP subnet exist without routable internet i.e. the mothership. There is a standard, "Wi-Fi Direct", that solves this - it's been around since 2009 and everyone implements it except Apple, so naturally nobody uses it.

Perhaps you mean p2p phone calls. Fine - although I have a hard time believing that the 900 Mhz radios capable of talking to a tower literal miles away are physically incapable of talking to each other. They're just programmed not to. In any case, the inconvenience and unreliability of sending files 3 feet, vs the streamlined experience of sending them via a corporate datacenter thousands of miles away, demonstrates that this is not about hardware. This is about software, and by extension about control.

wolvoleo

9 days ago

Bluetooth and WiFi on a mobile device don't have enough range for this stuff. You need to be too close to other participants.

Hence LoRa. Yes, 5G would be an option but the licensed spectrum (and firmware restriction) makes that impossible.

dTal

8 days ago

For the purposes of this thread, "this stuff" consists of:

>It's absurd that modern phones can talk to satellites hundreds of kilometers above, but not to other phones a few meters away in the same room, airplane cabin, train car etc.

Common everyday filesharing, not meshnet fantasies.

atomicthumbs

9 days ago

5G NR radios could also be capable of communicating peer to peer long distance if manufacturers (phone or baseband?) wanted them to. It's in the spec.

lxgr

10 days ago

Exactly: Perfectly capable in hardware, almost perfectly locked down in software.

throwaway270925

10 days ago

> share photos via Bluetooth across manufacturers and OSes

You can still do that! It never went away.

lxgr

10 days ago

It's never been a thing on iOS, unfortunately.

lichenwarp

11 days ago

Why is it that pretty much every ham operator I've met has been a complete jackass.

s03nk3

11 days ago

Is it so difficult to have schematics and pcb exported as PDF so that people do not need to fire up KiCAD to view the stuff?

mk_stjames

11 days ago

I encounter this problem as well when looking at projects on Github on a near daily basis it seems.

It just led me to finding this:

https://kicanvas.org/

worth a bookmark.

Retr0id

11 days ago

Or even a picture of the physical device! I found some on a linked sales listing: https://chelegance.com/products/kv4p-ht-plugplay-vhf-radio1-...

wolvoleo

11 days ago

Thanks! I was looking for that.

I see they use those cheapo Chinese RF modules (SA818). I've seen those also with SDR input/output, that's interesting. The underlying chip is very similar: The chip that will be used in this module is the RDA1846. This is a chip that's in most Chinese handhelds and is internally fully SDR but it decodes to analog. There's also the RDA1847 with similar pinout which offers the raw SDR stream and can thus be used for any mode, but with the added complexity of having to do the SDR decoding externally.

That means that this design could probably also be modified to do DMR. Though the SDR side might be a little bit too much to ask of an ESP32. On the other hand, it is only a very low bitrate signal.

oslem

11 days ago

Sounds like a great contribution you could make!

vitally3643

9 days ago

There are like six ways to export PDFs from KiCad. Project owners choose not to.

wolvoleo

11 days ago

Nice!! I wish it could do DMR though. Analog isn't used to much anymore in these parts. Although to be more precise, HAM radio has declined a lot but DMR repeaters are linked in groups so it seems like there's a lot more activity.

wolrah

10 days ago

Exactly my thought. APRS is nice but I'm just not interested in buying another otherwise analog-only radio. I know a lot of the popular digital modes are hard due to proprietary components but I'd be a lot more interested in something that supported digital voice and higher rate data modes even if it were just M17.

bdavbdav

11 days ago

I used to use SDR for DAB radio in the nexus 7 in the dash of my BMW E46. It didn’t work very well but was closer to being some kind of radio receiver (not trans at least)

RobotToaster

11 days ago

1w seems a little limited? A cheap baofeng is 8w.

Kaliboy

11 days ago

Those cheap baofeng's are illegal to use where I live on most of the spectrum they can operate on. Illegal to press the talk button anyway.

So maybe the 1w is also a regulatory issue.

souterrain

11 days ago

In the US all transmitting at 1W with these radios is contrary to regulations.

That said, no one is going to stop you from squatting on 146.580 MHz for example, a frequency commonly used by outdoors folks, rules notwithstanding.

ac29

11 days ago

> In the US all transmitting at 1W with these radios is contrary to regulations.

With a ham license the limit is 1.5kW, without one its zero

asdff

11 days ago

How does the FCC enforce this sort of thing? Are they listening in to certain frequencies nationally with the ability to triangulate a handheld down to actually identifying someone?

Bender

11 days ago

They mostly don't. They send scary letters and maybe eventually fine someone. They have a handful of triangulation vans for the entire US. The barely respond to complaints that are revenue impacting and generally don't respond to complaints from hams that a non ham is using their equipment. Once a quarter they make an example of someone so that it appears they enforce things. The people they make examples of are usually trying really hard to troll them or doing something highly disruptive and these will end up on a few websites and magazines.

People tinkering and staying away from ham bands will generally be fine and for the cheap ham gear that made easy by design usually by doing a factory reset or worst case having to clip a diode to widen their frequency ranges. Most ham gear is designed to be highly hackable.

gh02t

11 days ago

You only really get attention from the FCC if you interfere with some other service, especially broadcast or emergency communications frequencies. Grumpy Hams will also hunt you down sometimes, but only if you're persistent. Otherwise yeah, the FCC can't practically enforce every rule everywhere and doesn't really try. Still, get a license and be a good citizen if you want to play with this kind of thing, it's not very difficult.

Bender

11 days ago

Agreed. One youtuber [1] calls them Sad Hams. That has stuck with me. Linked video is the FCC's recent enforcement actions. I personally find some ham gear to be slightly less sloppy 11 meter rigs and a number of others agree.

[1] - https://www.youtube.com/watch?v=QeMjMlHpus8

martheen

11 days ago

Someone complained, they send someone to check and triangulate, verify that the operator doesn't have the license, then issue a warning or fine.

asdff

11 days ago

I wonder how close they can triangulate? I'd guess they are SOL if I am in an apartment building with 200 other units.

martheen

11 days ago

I mean, if you're deliberately being a dick about it, they will ask for a warrant and have the LEO accompanying them while triangulating, they can easily figure out which unit and even room you're in by walking around in the building with directional receiver.

But in general, yeah, unless you do it regularly, living near sensitive facilities (airport, military base, hospital, factories, research labs etc) or deliberately transmitting at/near emergency frequencies (police, paramedic etc) at most you'll get yelled by a pissed off operator (at that point, stop, they could be already coordinating with someone else to triangulate you)

user

11 days ago

[deleted]

l23k4

11 days ago

Unless you're going out of your way to force them to react, they do not.

lormayna

11 days ago

Only on the paper, the real power is around 3/3.5W

takipsizad

11 days ago

1w is usually okay and using 8w from a phone is probably way too much power demand

landgenoot

11 days ago

> 1 watt transmit can go miles yet sips your phone’s battery

How far can such a device reach in a typical urban environment with the longer antenna?

lxgr

11 days ago

VHF is effectively line-of-sight, and no antenna size can change that (although it does improve efficiency for both sending and receiving), so for two handheld radios, you are limited to about 10 km.

The only thing that really helps extend the range is elevating the antenna, and repeaters allow you to do that even between two mobile stations.

ac29

11 days ago

VHF doesnt need line of sight, it has excellent ability to penetrate obstructions due to its long wavelength. For example, its not difficult to receive FM radio transmissions indoors, even from tens of km away. Some obstacles will effectively block any radio signal though, such as solid earth or concrete (esp rebar reinforced concrete).

You are right that handheld radios wont get more than about 10km, but that is due to the curvature of the earth. Mountain top to mountain top, you could easily do 50-100km

lxgr

11 days ago

That's what I meant by "effectively": Reflections, diffraction, and (moderate, i.e. think doors and windows, not walls) light obstacle penetration make some indoor reception possible, but the radio horizon remains the strong limiting factor.

That's even true for the L- and S-band; otherwise you wouldn't be able to use a cell phone in a windowless room, for example. (Much of what's commonly attributed to "object penetration" is actually mostly due to reflections and diffraction around obstacles.)

giantg2

11 days ago

I think there's other phenomena that people might be interested in such as E-skip and tropospheric ducting. Although you cant really rely on those.

souterrain

11 days ago

Practical use, dense urban, flat, UHF is about 1km. VHF is worse.

Most radio amateurs would utilize a repeater to get over this limitation. Assuming there is a reliable repeater one is welcome to use nearby.

quietsegfault

11 days ago

In dense urban environments, VHF (~144 MHz) actually performs better than UHF (~446 MHz) for a few reasons... Lower frequencies diffract better around obstacles. VHF diffracts more readily around and over buildings than UHF. VHF penetrates building materials better than UHF — lower frequency = longer wavelength = better penetration through walls. UHF suffers from higher path loss over distance compared to VHF.

Melatonic

11 days ago

Is it still worth it to mess around with older full duplex handhelds ?

gh02t

11 days ago

Analog handhelds are still abundant, they've gotten smaller and more efficient but older ones are still basically just as good as new. IMO digital handhelds are superior, but digital protocols are much more fragmented so analog remains king as a common denominator (practically every digital handheld can do analog, too).

tamimio

11 days ago

I loved it, amazing work, thanks for sharing it!

Crunchified

11 days ago

This doesn't turn your phone into a ham transceiver at all. It turns your phone into a transceiver controller. Given that a cell phone is a transceiver, this headline is rather disappointing clickbait.

hakfoo

9 days ago

Arguably, that's still a viable thing.

I've used a few handheld transcievers and they tend towards clunky user interfaces-- tiny, fiddly displays, highly modal keys, and basically needing to memorize the manual to use it.

Even worse-- there's not a strong correlation between the quality of the RF unit and the user-interface. The Yaesu FT-60 is a reliable, high-quality radio, with all the user friendliness of a live porcupine.

But with a 6-inch pocketable touchscreen you could solve most of the UI problems, and focus on the radio problems.

Keeping the division also gives you flexibility-- you could design a 25-watt tabletop unit, or modules for different bands, that shared the same basic control software and UI

alexwwang

11 days ago

Agree.

We need a compact short wave transceiver device actually.

jonah

11 days ago

QRP is the keyword you may be looking for. 9W, battery powered, SDR shortwave transceivers. There are inexpensive and expensive versions.

alexwwang

9 days ago

The antenna is the most critical part in this system. The transceiver is the second part, as to me at least.

FabCH

11 days ago

Baofeng is 20 dollars? How much cheaper and compact do you need?

And I know, I know, Baofengs are notorious for going over the allowed noise limits… but still…

takipsizad

11 days ago

Baofeng's are not shortwave radios afaik

alexwwang

9 days ago

yes. it's a good choice for light use in UHF/VHF scenarios even communicating to satellites if you have a Yagi antenna.

jimnotgym

11 days ago

Baofengs also have terrible receive filtering. It is perfectly possible to hear no stations because you are being overloaded by something on another frequency. I tried my first SOTA activation with a Boafeng. A transmitter on another hill meant I received nothing, although stains could hear me. By a Yaesu, still cheap

sfmike

11 days ago

Is this prevented by physics or cost or just no one has the motivation?

gh02t

11 days ago

Compact HF/shortwave radios with transmit capability exist, but they're pretty expensive and are generally definitely portable but not quite handheld. The biggest user of such equipment is the military, so a lot of the tech is engineered for that with civilian/amateur use as an afterthought. ICOM, Yaesu, and Xiegu are probably the best known makers, and you're looking at ~$1000 as table stakes for a modern one, though there are some slightly cheaper options.

Handheld CB radios do exist and are cheap, but I've never really used them.

souterrain

11 days ago

There are a number of compact shortwave (radio amateurs prefer the term "high frequency" or HF, in contrast to VHF, UHF) transceivers. The impracticality is from the size of an efficient antenna.

I have personally made voice (single-sideband or SSB, which is analog like AM without wasting energy transmitting a carrier or redundant sideband) contacts with a 5 watt portable (Elecraft KX2) between countries in Europe, using a meter-long whip antenna and a trailing counterpoise wire.

These radios are incredibly complex weak-signal equipment, and that is reflected in the price.

That said, it is fun. Using morse code to do the same is even more fun.

I would never rely on this for off-the-grid communication, though.

RobotToaster

11 days ago

A compact CB transceiver would be fun.

topspin

11 days ago

Fun, but short range. A quarter wave CB antenna is about 2.7 meters long. Without that, you're making more heat than radio.

iberator

11 days ago

wspr is known to work at 7mhz band with just cloth hanger as antenna. i think its totally possible to use headphones as shitty longwave antenna too

topspin

11 days ago

Someone already pointed out how the WSPR anecdotes fail for CB. The longwave reception argument is fallacy as well. There, you receive powerful signals with a poor, receive-only antenna; the typical asymmetric model of commercial broadcast radio (and cellular, for that matter.) With CB, both sides are low power. Legally, that is. And neither antenna is on a huge tower.

Several handheld CB radios exist, with little loaded whip antennas. You can go buy one. You'll see. They work for talking between two tractors in a field or whatnot. Past that, not so much. Today, you're better off with license free UHF handhelds for that use case.

jimnotgym

11 days ago

Wspr is very optimised for weak signals, though. You wouldn't actually hear anything but noise.

iberator

9 days ago

jt8 would work. designed for ultra low db reception. We are talking here -20 dbm and chattable

lovelearning

11 days ago

I don't see it as clickbait since the realities of the Android ecosystem is a shared context.

Most people know that just about every Android phone has a restricted hardware design, not an expandable one.

So, "turn your phone into X" is bound to automatically evoke images of another device that plugs into the phone via common connectors like USB or the audio jack and an app on the phone to control that device. That's what the phrase means to most people in the context of Android.

"Turn your phone into a ham radio transceiver controller" is neither needed nor entirely accurate, because then people will assume it can control _any_ ham radio transceiver.

Crunchified

11 days ago

The article is chiefly about a radio circuit you can "build", plus some controller software that happens to run on an Android phone. Meanwhile the headline is 100% focused on describing something that your phone can be made to do (which you have admitted that it can't).

The two don't add up, and your apologetic analysis doesn't convince me otherwise. It's still clickbait. An Android cell phone has radio guts, and that headline is just gutless.

lovelearning

11 days ago

It's not "apologetic" and it wasn't meant to convince you but to refute your pointless pedantic nitpicking for other readers.

lxgr

11 days ago

"Turn your phone into a nuclear reactor (by plugging it into a wall outlet served by a nuclear power plant)"

developer786

11 days ago

Why do some people get so hung up about minor things in life. The OP has done a fantastic job, not just building it, but both the delivery and mechanics.

lxgr

11 days ago

I agree, it is a fantastic job!

The title is still misleading to me. Not terribly so – hence my not quite serious reply – but I was expecting something like https://timestation.pages.dev/, which actually transmits using included hardware only.

lovelearning

11 days ago

Very funny.

However, your analogy is not equivalent to, nor an example for, what I said. There's a difference between a phone's own USB/audiojack interfaces and a wall outlet.

mystraline

11 days ago

[flagged]

souterrain

11 days ago

Practically, this take emphasizes my point on repeaters. Each open active repeater develops its own subculture. (That said, the overwhelming majority of repeaters in the US are idle.)

You may not be welcome on any given repeater.

jimnotgym

11 days ago

The only hams I ever enjoyed talking to were SOTA chasers when I was the activator. Bit of an all day job, and therefore something I don't really have time to do.

kotaKat

11 days ago

"Ham radio is like Chatroulette in that you don't know which old man at the other end of the ionosphere you're going to end up talking with."