whats the best antenna for ground vehicle use ?
i have a fpv truck , i use a dragon link , rvosd5 , 900/1w vtx (with lo pass filter ) , --all crammed up inside a 1 ft area , on the roof of a traxxas grave digger monster truck .
i use 2 patch antennas into an oracal diversity controller . i use the dipole on the truck for video , and a dipole for the dragon link .
everything is crammed together , but works . the conditions for reception are poor ( all antennas low to the ground , line of sight is through brick buildings and woods , and the dl transmitter is below ground in a basement )
what antennas are better suited for for ground use ? - i cant elevate the antennas on the truck . all of my interference is what apears to be "multipath" on the video , the rssi on the dl get to a point where before it drops off to zero it bounces up to 10 % and cycles ( like something pulsing in the area @ around 433 mhz )
i looked at the radiation patterns of my set up , and it looks like im "blowing" 1/2 my signal into the ground . like a bagle sliced in half ?
A 1/4 wave ground plane has it's primary lobe of RF energy up at about 20-25 degrees from the horizon. But transmitting from underground and receiving from an antenna that's moving only inches above ground is really asking alot. Get some decent coax (low loss, large diameter) with proper connectors, and get an antenna up on the roof, attic, upper floor window, etc.
A sample can be found here, just follow the dimensions for the 445Mhz antenna:
on the 433 side , the real answer is to get the transmitter above ground ( like you say ) , on my rssi i go from 0 back to 99 % just by elevating it to 1 ft above ground ( easily another 3000 ft ) .
on the video , the base station is above ground .. - i forget where but i thought there was an antenna that had a radiation pattern like an inverted frustrum of a cone or realy wide patch , or wide shape of a "v"
id rather listen up than to ground .
is there a way to focus a dipole -- perhaps to flatten out the omni directional radiation pattern , perhaps build a home made dipole for the vtx and arc it instead of straight , maybe a wider flatter pattern would cause more of the usable area to be above ground ?
plan b ?
go back to a vee antena and "aim" it using my pan and tilt ( back up camera ) .
right now i get 2000 ft of video and control , the video is poor , but if i move around a bit it comes in crystal clear , the truck goes to where it goes , then " camps out " a bit the moves on . i can drive through most of the patchyness of the video ------ hey,.. can the rvosd send control to a servo to aim an antenna ?
ground plane worked , i really didnt think it would .
its better than the dipole .
the video is over all better , not even tuned.
the antenna is slightly less than 1 meter ( to obnoxious and spiny ) to make it taller .-- gets snagged on bushes.
cleaned up a slow gps lock .
then i tried an extention cable on the tx for the dragon link.
20 ft of 50 ohm , bnc on one end , a "nasty board mount sma jack" soldered in place on the other end , screwed the original antenna on and elevated approx 10 ft above ground --- aprox 1to15 %+ on my rssi . - i thought id do better.
i used a bnc adapter on one end to my dl tx , sma soldered to the other. --i could easily get rid of the soldered sma side (i suspect may be an issue ) and make my own 433 tx antenna ( next weekend )
the sma board mount is "open" to the air - at the point of connections there is no shield --you can see the solder joints . should i wrap it in foil ? -or try to shield it ?
did i loose that much in the cable ?
the dl tx runs cooler than with the antenna connected directly to it .
Last edited by raggedraider; 04-30-2012 at 12:20 PM.
A picture is worth a thousand words here. lol
When operating on 400MHz and higher, losses become larger with every connector inline as well as long coax runs.
Not sure what DL has for antenna connections (SMA or RP SMA??), but reducing the number of RF adapters will help. You can make a 1/4 wave ground pole by soldering some #14 gauge wire (of proper length) to the center conductor of the coax end, and 3 or 4 counterpoises (of proper length) to the shield. Bend the counterpoises at a 45 degree angle down.
At 433MHz, the lengths should be 6.48" for the vertical element as well as the counterpoises.
I'm not sure of the exact frequency DL uses, but that was just an assumption on my part. Here's an online calculator with various units of measure:
i cant find the EXCAT frequency for dl . but i have a dummy load , and rssi -- i should be able to tweak it untill i hit my max .
an antenna on the end of an extention cable , via converter connector ( sma to bnc ) is not the way to go , but in my case the gain i get by putting an antenna above ground - becomes greater than the loss of the cable .
on the truck side (vtx ) i have about 1 meter of cable then the antenna , im thinking that the same is applying ( elevation ) may have been some of the increase i saw in picture quality . as well as the type of antenna
the dl has sma male on everything . -- and the option to buy a longer ( 36 inch antenna that has one end crimped , the other easily cut ) - encouriging for novice antenna hackers .
the one "nasty" connection i have is an "open" to the air soldered connection , right at the point the antenna connects at the far end of the dl - tx cable . - what would be the recomended shield to cover that joint ? - if i thas any effect?
even worse the joint is a 90 degree type .
project for the weekend , build a ground plane for 433 . on to the end of the cable .
ill try to get pics , this is really a lesson in what not to do , in a 2-d world , all of the antennas are inside 1 ft of each other , dl , gps, vtx ( @ 1 watt ) - oddly i have no twitching .
thnx , yet again
433 ground plane resulted in a 40 % increase where the whip showed 0%
so a ground plane 433 antenna is good and has helped .
so much so that i managed to get in the wrong part of town and was jumped by a local , resulting in another 4 hrs of rebuild time .
the grey area is now , can i post a video of an underage kid attacking a fpv truck on youtube ? maybe commiting a crime ?