I can’t do this with meshtastic in the states because the legal frequency here is 915 megahertz
There are quite some ISM bands, but every country has its own regulations what is permissible on which band and exactly which frequencies may be used. What is 915 MHz in your country is 868 MHz here in Europe. Up to now my impression was that 433 MHz is a worldwide ISM band. May be I am wrong.
I just browsed the Mouser Web site for 915 MHz antennas. There are very few with more that 3 dB antenna gain. That is next to nothing as the 0 dB reference is a theoretical, really omnidirectional antenna that simply does not exist: Even a simple dipole has more than 2 dB more gain in its best direction.
If you search for “915 MHz gain antenna” you get quite some hits. Many are really commercial offerings without prices. An Amazon I found a 8 dB antenna that is 45" long and costs 100 US-$.
Which means you should search for “915 MHz gain antenna homebrew” and find something like Building an 868/915 MHz co-linear antenna for Lora TTN gateway - YouTube. Look at the comments and you know why I tried to push you to a commercial antenna.
I have another tip for you:
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Go to Funkeigenschaften der Homematic-Zentrale CCU verbessern. This is in my former commercial Web site.
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Ignore the German text and scroll down to the first two pictures. This in an antenna for 868 MHz that I built some years ago for my home automation system based in a Raspberry Pi. I measured that antenna!
There are two things to take away for you:
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Shorten the dimensions in an 868/915 ratio. Higher frequency means shorter wavelength.
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Look at the coil below the antenna. The ferrite ring was from my junk box. If you have nothing else, even a plastic ring might help.
The comparison is far fetched, but the theory behind it is quite complicated: The coil works like the muffler in your car. It reduces the exhaust noise. That is to mean: The digital electronics in your LoRa device produces wideband electronic noise. If that is coupled into the antenna, the sensitivity drops. Your LoRa unit cannot hear its quieter partners.
This is valid for most digital electronics unless it works with extremely low clock frequencies like a clock (0.032 MHz) or an Arduino system (16 MHz). I use it everywhere, for example in my shortwave mobile station, see Mobilbetrieb mit Magnetfuß-Antennen, picture at the bottom. The last example works for the 7 to 30 MHz range and prevents all the electric noise of my car to enter the antenna. The difference is between not being able to hear any ham radio station to make a connection from Europe to Australia.
…and another, perhaps a little confusing topic: “effective radiated power” (ERP)
It is not by chance that most LoRa units deliver 100 mW (20 dBm) or so. And it is not by chance that most antennas for 915 MHz are low-gain.
At least over here ISM devices are limited to 100 mW ERP. That means that you may feed 100 mW of radio frequency power into an antenna with a gain of 0 dBi.
The next thing to explain is dB in the three variants I used here:
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dB by itself is only a “logarithmic” unit for ratios - mostly power ratios. This makes many calculations much easier as you can add or substract instead of multiply or divide. 0 dB is a ratio of 1:1. 3 dB is 1:2. 6 dB is 1:4 and 10 dB is 1:10. Now it is easy: 20 dB is 1:100.
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dBx is a ratio to a given nominal value. 0 dBm is 1 mW. 0dBi is the gain of an “intrinsic” antenna that radiates in all direction evenly. If you feed 0 dBm of radio frequency power into an antenna with 0dBi gain you produce an ERP of 0 dBm.
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Why is that important? If you must not exit an ERP of 20 dB and use an an antenna with 8 dBi of gain you must reduce your transmitter power to 12 dBm. You might be allowed to add 1 or 2 dB of power as you have some loss between the transmitter and the antenna. Lets add 1 dB, so you may set your transmitter to 13 dBm which is 10 dBm + 3 dB = 10 mW * 2 = 20 mW.
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Why should you then bother with a 8dBi antennas at all? Because the antenna makes your receiver more sensitive by 8 dB. In normal words: With that antenna you “hear” your partners as if they had 8 dB more transmitter power.
The disadvantage is that each station needs such an antenna so they can still hear each other. Otherwise you could improve your whole mesh network by updating your hub to such an antenna.
Howdy, I appreciate all the information. Trying to figure out where I got my information from previously that seemed to contradict part of what you were saying and I found this calculator:
https://www.everythingrf.com/rf-calculators/eirp-effective-isotropic-radiated-power
If you put in the 20dbm for the meshtastic radio, 0 for loss since the antenna would be connected directly to the radio, and 8 for an antenna with 8 dbi gain - it comes up with 28dbm which is under the allowed 30dbm or 1watt max, so how does that compare with your information? Math is not my strong suit and my understanding of yours and anybody else’s explanation of the differences between DBI, DBM etc is weak at best, so is this calculator incorrect or am I looking at it incorrectly, because if I’m hearing you right it seems you’re saying this configuration would be over 1watt unless I were to somehow reduce the transmit power of the meshtastic radio which as far as I know I cannot do?
I calculated with an allowed ERP of 100 mW. Otherwise the calculator uses exactly everything in exactly the terminology I used. If you may use 30 dBm (1W) - even better!
This also means that you need no power hungry 1 W amplifier.
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I hope DL4NO does not mind, but I’d like to call people’s attention to this persons profile, we’ve chatted a bit and they have a lot to share as you can see by their profile introduction, including open source software for a similar project:
I am a radio amateur involved in emergency communication. I see great potential for LoRa, especially in emergency welfare traffic.
With nearly 50 years of radio communications experience I might be able to help in specific areas. Much of this can be used for ISM systems like LoRa.
As radio amateurs we can use 433 MHz equipment beyond the ISM limitations, e.g. higher transmit power than 100 mW (20 dBm) or directional antennas. The downside is that ham radio traffic must use “open language”, so everyone with the right equipment is able and allowed to listen in.
I will try to introduce here 40 years old technology: FIDOnet - I was a FIDOnet node at that time, address 2:246/14.
FIDOnet was a communication network using extremely expensive and slow phone calls. They developed means to make maximum use of the available bandwidth. My first modem did 2400 bits per second.
Much of the FIDOnet software is open source, mostly written in C. There also are technical documents that should still be available on the net. There is no need to re-invent the wheel!
Joined
3 days
For the FCC regulation about ISM band in the US, maximum transmitter output power, fed into the antenna, is 30 dBm (1 watt). Maximum Effective Isotropic Radiated Power (EIRP) is 36 dBm (4 watt).
Thus you could fed 30dBm into an antenna with gain of 6dBi.
Reference:
If you have Amateur Radio License, you could go up to 1500W.
Reference:
https://uniteng.com/wiki/doku.php?id=meshtastic:station#operation_with_amateur_radio_license
433mhz doesn’t cover many regions including the US. See “Frequency Allocations”
So you have the information valid for you and you can understand it by distinguishing between output power and ERP - even the additional EIRP 
BTW: After reading more about Meshtastic I get the feeling that all that FIDOnet technology is not what the Meshtastic project is all about: They think of a meshed broadcast system for a small group whithout much fuss. You set a single LoRa channel for all devices of the group and that’s it.
So I think I should search for other LoRa project that might be interested.
I think you described quite well what I also think about Meshtastic: easy to set up for smaller groups. It is then up to use-cases how it get used. I plan to have a small meshnet with telemetry instruments positioned quite close to each other.
If I need more distance I can always revert to LoraWAN-stack from TTN as I have also a LoraWAN gateway and some MQTT-plugins available. My record with 16dbm is over 100 km (Heidelberg - Frankfurt), but then you need to be quite high and you need other gateways available. Meshtastic is autonomous in that sense.
So, what kind of usecase you have in mind where you like to use LoRA?
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I certainly know about how ISM technology works. It is well known that you can interrupt WLAN systems with microwave ovens: Both use the 2.4 GHz ISM band. WLAN uses 0.1 W while microwave ovens typically use 800 W. So keep the door sealings clean!
Over here we have a 433 MHz ISM band and thee is LoRa equipment for this band.
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My usecase is emergency traffic. Every power outage creates a “black hole” that hardly any information can leave. My idea is a LoRa mail system that even works where WLAN etc. don’t.
My feeling is that standard Internet technology needs too much data traffic. Therefore I would like to use 40 years old technology relying on store & forward methods with a minimum of data traffic on the physical layer: FIDOnet was developed with 2400 bit/s modems in mind.
In the 1990s I operated a FIDOnet node (2:246/14). FIDOnet evolved from modems to ISDN to TCP/IP. It should not be extremely diffcult to create a LoRa interface.
We could set up a hub at an elevated location. Either the system works locally or it gets Internet access. Simple nodes could be set up with a solar power supply and providing a WLAN. Upon entering this WLAN with a smartphone or so you are forwarded to a Web server in this node whose contents can be modified with messages sent through LoRa.
A webmail GUI could allow to create a mail accout. Each user is entered in a public user list that is updated and distributed all over the system. I would even propose that accounts might be created in the name of a person where the messages are forwarded to another person.
This way every person in a cut off village can be reached through this system, even old people. The authorities could use this system to find missing people. If the hub has Internet access, even mails in and out of the desaster region would be possible.
You can reach me at dl4no (at) dl4no.de.
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In the meantine I searched on other fronts:
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The Austrian radio amateurs have started a LoRa system called MeshCom.
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This weekend a proposal of mine was published in the Fidonews. That is the weekly newscast within the FIDOnet. Let’s see whether someone ist interested there.
Accordung to the Fidonews editor not much is happening in FIDOnet anymore. That is quite understandable: Why should you add another layer over the Internet with some specialized software? With a transport layer using LoRa FIDOnet could go where Internet cannot.
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Anything we can do to make these systems more widespread - feeling I have relied to heavily on the internet and need to be prepared for it to be taken away or at the very least more heavily restricted since it’s centrally controlled.
LoRa does not match up great with IP networking, seemingly low bandwidth options like IRC and FIDOnet exceed the bandwidth we have available pretty quickly.
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I have operated a FIDOnet node for more than a decade when modems were the way to go and see it differently: For routine operation LoRa is slower than nearly all alternatives. But if there are no alternatives, FIDOnet over LoRa could be a life saver!
My basic idea was disaster relief. And do not throw Internet technology and FIDOnet technology in the same basket: For a given bandwidth, FLDOnet can easily transport ten times as many short mails as SMTP and the other mail protocols in the Internet.
The basic difference: In the Internet you need lots of handshake sequences for every single mail. FIDOnet puts for example 100 mails in a single compressed file and transfers that with a handshake at the start and at the end of this file transfer. Add to that the effect that file compression works better for larger files.
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LoRa is no transport medium for higher net levels. But I see possibilities at the very basic level as its infrastructure is much simpler and much less power hungry than nearly everything else.
Compare it to satellite phones: They operate at 9600 bps, if I am informed correctly and their usage is extremely expensive. But for example sailing vessels use that technology routinely to get weather data.
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For devices in the US operating on the ISM band it is not only the maximum EIRP that has to be met but also a maximum PSD (Power Spectral Density) of +8dBm. Although unless you are trying to get a device FCC certified now one is really going to know the difference.
The PSD requirement is basically there to makes sure that the +30dBm signal is spread out somewhat uniformly over a somewhat wide bandwidth. This requirement is particular to devices that are not frequency hoppers. Semtech has some great whitepapers on it.
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As some have mentioned in other replies there are RF combiners that you can use to combine two 50 ohm sources. However when connecting antennas of two different types like that you may actually degrade performance because of the phase differences in the signals from each antenna causing some cancellation in the signal.
Combining antennas is a practice frequently used in amateur radio and CB but when used in that sense is done with co-phasing harnesses and antennas of the same type. The co-phasing harness consists of a T connector and typically around a quarter wavelength of coax from the T to each antenna. The quarter wavelength is the electrical length not the physical length. The electrical length takes into account the velocity factor of the cable.
In addition to the co-phasing harness the antennas are also physically spaced a specific distance apart and if using directional antennas they are pointed in the same direction.
Depending on the type of directional antenna you use and how narrow it’s beamwidth is you may get enough coverage off the side lobes of it for the people close to it to be able to connect while still reaching the distant station.
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