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Feb 12, 2011, 03:03 PM
Registered User
Question about 72mhz antenna length (yeah I know, not that question again...)

As recommended reading material by Ben (I think?) I was directed to a link showing Trappy's early setup in which he had a 4 meter RC RX antenna for his long distance setup at the time. At some point jumping through search boxes and numerous other threads, I came across this:

http://www.rc-cam.com/ant_exp.htm

scroll down to see the chart on length vs gain

For his tests, I assume that his use of 0dbi at the 40" length is nothing more than a reference point as a 0dbi gain antenna would be a perfectly spherical radiation pattern. His use of 0dbi is just a point in which gain improvements or lack of can be measured. He makes a point to note that at the 62" length showed the highest gain increase. Now, I'm still not totally grasping the concept of impedence other than matching the RSSI at the Tx end and the Rx end will give you the best range. Although in his test, the 62" length offered the highest gain, it might not be at the desired impedence. Correct? I was under the assumption that all changes in antenna length are done in 1/4 wave increments to maintain the correct impedence, which is proved in his chart as the RSSI values at 40" and 80" are nearly identical. The 62" length is approx 3/8 wavelength.

Or am I overthinking this again, is gain simply a derivative of voltage and if the RSSI values increase the theoretical gain is increased but that means nothing if the impedence doesn't match between Tx and Rx? If that's the case, what is the point in making an antenna longer than 1/4 wavelength if there is no gain increase? Obviously trappy must have been on to something to hang a full wavelength antenna off his plane because that had to be a nightmare to keep up with lol I have a hard enough time trying not to step on my 40" antennas
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Feb 12, 2011, 04:53 PM
KC6ZZS
Ben Clerx's Avatar
Quote:
Originally Posted by crash821
Ben,

That's a lot of good points, I appreciate you taking the time to respond. I guess I didn't mention it in this thread, must have been another, but my decision for 900mhz was because my step-dad is in the process of putting a 2.4ghz system together. I was going to put something different together so that we can get an idea of the best frequency for our area. His goals are different than mine, he doesn't plan to do any long range, mostly line of sight stuff. I plan to start small but work my way up to long range stuff. I think what Trappy did with 2.4 ghz is great and I'm sure it can be replicated in certain areas of the US but I just have a hunch that I'm better off sticking with 900 or possibly 1.3 for the area I'm at. There's just no way of knowing until I get something running.

Thanks for the tips on the 12V power for the Vtx system. Like usual, I overthink things too much. Hard part of having an engineering mind. I will just keeo in simple, run the video system straight off a 3S pack and do several tests to monitor what kind of actual flight time I can get. My only concern with this setup is that when voltage drops, so does Vtx output power, which means when you are 5 miles out and you start to turn toward home, you are already losing video range and the whole trip back home, power keeps going down making the whole flight back a constant gamble of enough power for a clean signal. Or am I just over thinking it again?

Eventually, once I get a setup going I planned on building my own antennas and experimenting with different setups. I don't see why making 900mhz antennas are any more difficult than 1.3 or 2.4, as the need to be more precise increases through the frequency range. Correct?
900 Mhz antennas would actually be easier to make. it's when the freq gets above 2.4G that it gets critical. The antenna elements are usually a quarter wavelength corrected for the type of antenna you're making. At 5.8 G, a quarter wavelength is small enough that a 1 mm error in length will have a measurable affect on range.

An Aero degree with some EE is a good choice. Checkout the UAV field. Once there, get an MBA. Managing engineers will get you a better paycheck than just being one.
Feb 12, 2011, 05:16 PM
KC6ZZS
Ben Clerx's Avatar
Quote:
Originally Posted by crash821
Question about 72mhz antenna length (yeah I know, not that question again...)

As recommended reading material by Ben (I think?) I was directed to a link showing Trappy's early setup in which he had a 4 meter RC RX antenna for his long distance setup at the time. At some point jumping through search boxes and numerous other threads, I came across this:

http://www.rc-cam.com/ant_exp.htm

scroll down to see the chart on length vs gain

For his tests, I assume that his use of 0dbi at the 40" length is nothing more than a reference point as a 0dbi gain antenna would be a perfectly spherical radiation pattern. His use of 0dbi is just a point in which gain improvements or lack of can be measured. He makes a point to note that at the 62" length showed the highest gain increase. Now, I'm still not totally grasping the concept of impedence other than matching the RSSI at the Tx end and the Rx end will give you the best range. Although in his test, the 62" length offered the highest gain, it might not be at the desired impedence. Correct? I was under the assumption that all changes in antenna length are done in 1/4 wave increments to maintain the correct impedence, which is proved in his chart as the RSSI values at 40" and 80" are nearly identical. The 62" length is approx 3/8 wavelength.

Or am I overthinking this again, is gain simply a derivative of voltage and if the RSSI values increase the theoretical gain is increased but that means nothing if the impedence doesn't match between Tx and Rx? If that's the case, what is the point in making an antenna longer than 1/4 wavelength if there is no gain increase? Obviously trappy must have been on to something to hang a full wavelength antenna off his plane because that had to be a nightmare to keep up with lol I have a hard enough time trying not to step on my 40" antennas
You're starting to over-engineer things again. No problem with that as long as you're on the right track. If you're on the wrong track, over-engineering makes things worse.

Direct your antenna questions to IBCrazy. He's the expert. I think Trappy simply made a quarter wave trailing antenna. 1-inch accuracy is fine. All the 72 MHz stuff is such broadband that you have plenty of leeway (I don't think anyone knows what the real impedance of that stuff was anyway). Also, in Europe they use 35 MHz. So, I don't know if he was on 35 MHz or 72 MHz, and that would make a difference in the antenna length (but stock antennas weren't much different in length between 72 MHz and 35 MHz, which gives you an idea how bad they were, while still performing well--likely with some internal tuning).

Back then, for our competition sailplanes made out of carbon fiber, we ran about 12-18" of antenna out the tail (the internal part technically not contributing to antenna length) and we probably had 2 mile range capability.

For 72 MHz, you're probably better off field testing than trying to engineer it. Make an approx 1/4 wave antenna, set up the plane and get up a hill with the Tx and have a buddy take the plane 1,1.5, and 2 miles away and see if you have control with the other electricals turned on (comm via cellphone). You could use RSSI in the air, but a real ground range test will give you an idea of what to expect in the air. If it doesn't work well on the ground, it ain't gonna work much better in the air. Don't loose the plane. Video record all the flights on an OSD page that displays GPS position. Get a cheap hiking GPS that you can enter GPS positions into in case you have to join the "FPV Hiking Club". By the way, a spotter is worth his weight in gold....or at least beer. He'll also keep you from doing something stupid (the learning curve will be steep at the beginning).
Feb 12, 2011, 06:14 PM
Registered User
A few years ago I built UAVs for Griffon Aerospace, specifically the Outlaw. It was fun but being on the design and flight side of things would have been a lot more fun.

Guess I was overthinking the antenna stuff. Brain was suffering from over-geekage. I figured field results would answer my questions the best, I was just trying to eliminate as much troubleshooting as possible later. I will just plan on getting one of the corona synthesized receivers with 40.5" long antenna for ch54 and have a friend start driving. Is that pretty much all you can do without going to UHF? Also does gauge of wire that I replace the antenna with matter?
Feb 12, 2011, 06:18 PM
Registered User
I won't have OSD or GPS for a while since that's just as much of an expense as the video setup. Because of that I won't be flying very far out. If I ever go out of line of sight without gps it will only be an area I am very familiar with that's unpopulated with a stable aircraft like a slow stick with some kind of locating beeper on it. Too bad gps alone is expensive. All the other OSD stuff is cool but GPS is the most important to me.

I haven't seen any direct comparisons between the dx201 and the sn555. Any suggestions? Or is the CCD killer the best bang for the buck?
Feb 12, 2011, 06:57 PM
Praying for better weather
Coyote64's Avatar
As long as the wire is the same diameter or thicker is fine.
Feb 12, 2011, 08:51 PM
Registered User
I got plenty of 22 and 24 gauge laying around. Wasn't sure if that would be too thick considering the wire that typically comes on Rxs is 28gauge or smaller. Thanks Coyote!
Feb 12, 2011, 08:55 PM
Running jokes into the ground
b-29er's Avatar
nvm
Last edited by b-29er; Feb 12, 2011 at 08:55 PM. Reason: wrong page
Feb 12, 2011, 11:33 PM
KC6ZZS
Ben Clerx's Avatar
Quote:
Originally Posted by crash821
I won't have OSD or GPS for a while since that's just as much of an expense as the video setup. Because of that I won't be flying very far out. If I ever go out of line of sight without gps it will only be an area I am very familiar with that's unpopulated with a stable aircraft like a slow stick with some kind of locating beeper on it. Too bad gps alone is expensive. All the other OSD stuff is cool but GPS is the most important to me.

I haven't seen any direct comparisons between the dx201 and the sn555. Any suggestions? Or is the CCD killer the best bang for the buck?

I have a DX201. Very nice picture (RangeVideo.com has them). Downside is the $150 cost. Get it as your second or third camera. For your first attempts, get something a little more "expendable". Try these:

securitycamera2000.com # PZ0104 Good picture, good low light ability, not configurable via OSD, but has a metal case to reduce noise. $43 free shipping (it takes a few weeks from China). This would be my first pick for you. A lot of bang for the buck. Use the savings to get a DX201 later when you're less likely to "lose" it.

#PB0191 Has an OSD, Good day camera, no case (just wrap it in tape or heatshrink). approx $70

#PZ0236 Also a "board camera" (no case). Good day camera and very good low light capability (starlight/moonlight). About $70

Back to the stuff you can buy in the USA (still made in China):

RangeVideo.com:
DX201 we talked about it $150
KX191 2nd best at $105 Excellent color and pretty good low light

NGHobbies.com
KX171LV Thats the one you can use from 5-15 volts. It's also nice and small. And it's a good value at $65

Well, thats all I know about cheap cameras. My requirements are mainly for night ops, thus the low light cameras. There's more in that department in the $150 range, but you won't need that. Just bought a thermal camera the size of the DX201, but that cost several thousand (thats the only way to do night ops right, but thats another story/project).

Daniel Wee did some camera tests here:

http://forum.tsebi.com/viewtopic.php?f=4&t=109

Anyone else with some good camera reviews?
Feb 12, 2011, 11:43 PM
USA: LakeGeorge, New York
Ben Clerx

I have this one and its awesome !

http://www.readymaderc.com/store/ind...roducts_id=207
Latest blog entry: My FAVORITE FPV CockPits'!!!!
Feb 13, 2011, 01:13 AM
Registered User
Well I plan to get a Go Pro HD (happen to have a $100 off deal) for getting nice recorded video for later playback. I know it can be an option to use for live output with the new firmware update and I still haven't been convinced by anybody to not use it for live output. Since I'm going to be getting the Go Pro anyways, can somebody convince me why I shouldn't use this for my inflight camera too?

Now, I don't have to buy the Go Pro right away, my discount lasts until the end of the year so if I do hold off on it, having a camera that displays GOOD video in MOST lighting conditions is a MUST. I hate the videos that show the sky all nice and pretty and the ground is a dark smudge or vice versa when looking at the ground, ground is clear, sky is white. I need to be able to see where I'm going and a camera clear enough to be able to recognize terrain features, not just green ground, blue sky.

#PZ0236 sounds like a good option. I like the fact that its good low light as I would like to eventually try out some night/parking lot flights.
Feb 13, 2011, 01:31 AM
Registered User
I dunno man, after having watched so many dx201 videos in the past and after a few more I just found, its really hard for me to not get this camera the first time around. Yeah its more expensive, but I'm one of those guys that its just worth it to do things right the first time...hence the million questions I'm asking. The dx201 does exactly what I want with no other rivals within its price range or cheaper. I think I would be happier with a NICE fpv camera and just getting the Go Pro HD later and adding it to an already proven setup.

The first sample video just says it all

http://www.rangevideo.com/index.php?...roducts_id=158

Whats with the two ground wires on the dx201 (scroll down on the page from the above link)

Last question for the night:

Is there anything wrong with having the Vtx and the camera mounted in close proximity or even back to back (like you would find something mounted on a 360 degree swivel)? Not only would this keep the wires shorter (less wire to get outside interference from) but would also create a portable unit for use on other planes, etc.
Last edited by crash821; Feb 13, 2011 at 01:40 AM.
Feb 13, 2011, 08:58 AM
KC9TPL- Get Legit
BloomingtonFPV's Avatar
I'm not sure why people love the DX201. I flew with it for a while but when you looked in the direction of the sun it always looked like a nuclear explosion. Since I tended to fly in the evening this was a real problem.

All my planes now have this:

http://www.readymaderc.com/store/ind...roducts_id=163

which is smaller, lighter and has a great OSD menu. Plus it is $50 cheaper...

I didn't like the WDR600 camera either. I sold my DX201.
Feb 13, 2011, 11:17 AM
KC6ZZS
Ben Clerx's Avatar
Quote:
Originally Posted by crash821
I dunno man, after having watched so many dx201 videos in the past and after a few more I just found, its really hard for me to not get this camera the first time around. Yeah its more expensive, but I'm one of those guys that its just worth it to do things right the first time...hence the million questions I'm asking. The dx201 does exactly what I want with no other rivals within its price range or cheaper. I think I would be happier with a NICE fpv camera and just getting the Go Pro HD later and adding it to an already proven setup.

The first sample video just says it all

http://www.rangevideo.com/index.php?...roducts_id=158

Whats with the two ground wires on the dx201 (scroll down on the page from the above link)

Last question for the night:

Is there anything wrong with having the Vtx and the camera mounted in close proximity or even back to back (like you would find something mounted on a 360 degree swivel)? Not only would this keep the wires shorter (less wire to get outside interference from) but would also create a portable unit for use on other planes, etc.
Get the $43 camera and use the $100 bucks you save for a GoPro, OSD/GPS or a DX201. Boomingtons camera is probably also a good choice. I like the Super HAD and HAD II chips, so I might have to try one. I like my 201, so get a DX201, but you really need the OSD/GPS, even if it means a cheaper camera. The good video will come from the GoPro anyways.

Separation is good. I don't mean from your wife/girlfriend. I mean from other FPV components. Noisy cameras will be in the front for the view. R/C receivers usually mid section and I like the Tx in the tail or (out in the wingtip in certain applications). GPS usually upper fuselage near wing leading edge. DCVAC.com low pass filter really helps a lot for the noise near the 1575 MHz GPS frequency as well as the harmonic on 2.4G if you're flying with that.
Feb 13, 2011, 12:43 PM
Registered User
I plan on putting together a pusher setup. That way all the video stuff is in the nose isolated from everything else and the motor and esc, rc rx, and servos are mid fuselage with at least 12" of seperation or more depending on the airframe I go with. One of the UAV platforms I have been designing would actually give me closer to 3 ft of seperation.

I guess the only real question I have left is the proper orientation of RC Tx antenna and RC Rx antenna. I assume your standard RC Tx and Rx antennas are omnidirectional vertically polarized. If thats the case, the best real world solution would be to have your RC Tx antenna sticking straight up and you RC Rx also sticking straight up out of the plane. Since a 40" pole on top of the plane isnt practical, you would run it out the wingtip, which means your RC Tx antenna should also lay horizontal. And if the RC Rx antenna is going to coming out of the plane horizontally, is it best to run it out the wing tip or down the fuselage? Also heard people having half of the antenna going down the wing and then letting the other half hang back in the wind, creating a Vee type antenna. If this was such a good choice, why not just use a real Vee for the RC Rx? (I know that these questions are best directed to IBCrazy but for some reason my PMs dont work and I'm not sure if the forum-sent emails worked either)

Anybody want to convince me not to use a Go Pro as a stand alone camera to record and live output at the same time?


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