Multipath Interference

[transceiver1]

This is not intended to be an FCC thread but more of a functionality thread.
If I were running 2ft or 4ft wavelength RF, line-of-sight, would I expect to have problems with multipath interference from trees/barns/hills/cliffs/buildings/ground/etc.
edit: This is data being sent, not just voice communications. Data must be almost 100% reliable.
edit2: 0.5mile or less distance between mobile and base.

 

[waltr]

Well 2-4ft wavelength is around 300MHz which is VHF/UHF.
This will or can have multipath interference.
Probably not a problem if TX in a rocket and RX is out in a field.

If you use a directional antenna (Yagi) then multipath will be much less (ant rejects RF from other directions).

 

[transceiver1]

Well 2-4ft wavelength is around 300MHz which is VHF/UHF.
This will or can have multipath interference.
Probably not a problem if TX in a rocket and RX is out in a field.

If you use a directional antenna (Yagi) then multipath will be much less (ant rejects RF from other directions).

I cannot help but wonder if there is some sort of solution/trick to the multipath problem (in addition to using a beam/Yagi antenna). I see people fly RC helicopters inside of concrete buildings all the time (granted not the same RF wavelength as mentioned above).
I have also seen RC helicopters fly from outdoors, through an open window of a building, and then back out of the building through the same window (line of sight was maintained the whole time).

 

[jderimig]

Not if you're standing still or moving slow. TV's worked fine for a long time at those frequencies in horrible multipath environments with crappy antennas. Data transmission reliability is from a link budget calculation. Data rate is the big driver, data encoding and error correcting logic (retry rules) is another level. Engineering is tradeoffs.

 

[waltr]

I cannot help but wonder if there is some sort of solution/trick to the multipath problem (in addition to using a beam/Yagi antenna). I see people fly RC helicopters inside of concrete buildings all the time (granted not the same RF wavelength as mentioned above).
I have also seen RC helicopters fly from outdoors, through an open window of a building, and then back out of the building through the same window (line of sight was maintained the whole time).

Yes, even though Multipath is real it typically does not impact the reliability of the link enough to notice. Modern RC is running at 2.4GHz. This RC systems are very reliable.

A well designed Data Link would negate most MP issues.
Also, since I assume this is Rocket to Ground and out in a huge field, there will not be much to reflect the RF to cause MP.

 

[jsdemar]

Yes, even though Multipath is real it typically does not impact the reliability of the link enough to notice. Modern RC is running at 2.4GHz. This RC systems are very reliable.

RC links at 2.4GHz are using spread spectrum with coding that reduces the effects of multipath. A delayed signal at the receiver will likely be out of sync with the code sequence and appear near the noise floor. Same with cell phone links. The magic of orthogonal code sets and mathematical correlation.

 

[gldknght]

Also, if you do get multipath interference at your receiver, try moving the receiver a few feet. You might step out of the multipath convergence area.

 

[waltr]

Yes, moving RX antenna a few feet works.

Also, if using a directional antenna (Yagi) will help reject multipath (RF coming from side of antenna).

 

[cls]

I were running 2ft or 4ft wavelength RF,

Could you clarify this? Amateurs don't call it 2 foot or 4 foot wavelengths. What frequencies exactly are you talking about? How are you going to xmit without a license?

 

[transceiver1]

Could you clarify this? Amateurs don't call it 2 foot or 4 foot wavelengths. What frequencies exactly are you talking about? How are you going to xmit without a license?

about 253.5Mhz or 507.1Mhz. the usual c/f calculation, c = speed of light, f = frequency.
Like I said, I do not intend for this to be an FCC thread.

 

[cerving]

I'm assuming that since you were speaking theoretically in your OP that those frequencies are not ones you're actually using. You'd need to use frequencies that are actually available for public use... closest would be 2m (about 150 MHz), 1.25m (about 220 MHz), and 70 cm (about 430 MHz). Use of those bands in the US requires a Ham license.

 

[gldknght]

about 253.5Mhz or 507.1Mhz. the usual c/f calculation, c = speed of light, f = frequency.
Like I said, I do not intend for this to be an FCC thread.

Fcc thread or not, if you try to transmit on Ham frequencies without a ham license at a club launch, there almost certainly will be a ham licensee there, and they will most certainly stop you from trying this.

As rocketeers, we are required to obey federal laws when flying rockets. That includes Fcc rules, too.

 

[cls]

about 253.5Mhz or 507.1Mhz. the usual c/f calculation, c = speed of light, f = frequency.
Like I said, I do not intend for this to be an FCC thread.

Thanks for confirming our suspicions.