38 replies to this topic

[Vrflyer]

Can you tell me why those antenna can be use on 900mhz and also on 1800mhz. Is it something related to half wave and full wave. Can I use them with my 200mw 900mhz tx

http://www.dealextre...ils.dx/sku.5323

Edited by Vrflyer, 28 February 2009 - 06:33 AM.

[Mr.RC-Cam]

Low cost dual band antennas are ~1/4 wave monopole designs. They will usually have typical performance on one of the rated frequencies, and much less performance on the other. So they are not all magic.

You could use it with your 900MHz A/V Tx. For our application, a standard 1/2 wave dipole on the Tx is usually a better performer, so I would recommend that instead. But at 900Mhz the good ones are a bit long. The little one you are looking at would be fine if you want something small and don't care about long range. But don't expect the same performance.

[Vrflyer]

Thank Thomas

[Vrflyer]

I installed my 900mhz tx in my Funjet plane this weekend and I had got jitters in my panning servo. The problem was cause by the new antenna, by replacing this antenna with the longer one, the jitter near completly gone.

Edited by Vrflyer, 02 March 2009 - 04:03 PM.

[Mr.RC-Cam]

That is a common complaint with the small 900MHz monopole aerials. A decently designed half-wave dipole antenna will often cure servo jitter and GPS interference problems. The L-Com HG903RD is nice, but quite big.

If you are using the half-wave dipole now and you still have some panning servo jitter, then replace the servo. Many of the imported economy types will get jittery near the 900MHz Tx, so you may have to try a couple to find a good one. Hitec HS81 and HS65 work well in 900MHz installations.

[Vrflyer]

I received this one from NGhobbies, it's big and heavy, my 200mw tx look so small near it...
http://www.nghobbies...products_id=221
what do you know about this antenna Thomas

Yesterday after I resolve my problem of jitter, I got another problem, with my video tx this time. The transmission was very poor quality sudenly, not even able to see any picture, with or without camera plug. I search long time never expect to found a so bad problem comming from my long antenna that was working perfectly one hours before. After replacing it with standard small antenna that come with the tx, everything was fine.

I disassemble the antenna cover and check for continuity and everything look fine, I also check for insulation between central pin and ground and I had infinity, but i tested it with cheap multimeter at home, I will bring it at my job for better testing. I can not see any defect at the pivot point, this antenna is near new, but i bent it around 20 time since new at the pivot point. If I find nothing, I will open it at the pivot point, I see only there the problem can come, the cable has been crush probably. Bizarre.

[Mr.RC-Cam]

The big one you have is the L-Com HG903RD. It is a dipole and a very good performer. Perhaps the best around for typical use. But it is a big fellow and can look a bit over-sized on a small model.

That is very strange that it stopped working after an hour of use. But it might not be the antenna. Instead, it may be the SMA's PCB connection inside your 200mW transmitter. The center conductor's solder connection may have broken away from the PCB copper trace due to the mass of the antenna and the stresses while flying. Perhaps the break makes better connection when the lightweight antenna is installed because the Tx chassis does not flex as much from the weight of the bigger antenna.

Another decent performing ducky is this one (from the store I work at): Compact 900MHz antenna. It's nice in that it is small (5.6 inches long), yet is a real half-wave dipole. The short length is because the driven element is a helical design. It is ideal for those that need something shorter than the big L-Com 900MHz dipole. Seems to do very well from the user feedback I have received.

[Vrflyer]

I need others 900mhz antennas, I will order three of your suggest antennas. Thanks

[Mr.RC-Cam]

Be sure to keep the big L-Com dipole for use on the video Rx. For a step-up in Rx performance, a well designed 6-8dBi patch antenna is nice.

[Vrflyer]

The story is not finish...

You think it's perhaps a trace on the pcb circuit that is broken, but I got two new 200mw tx and none had time to fly yet. I swap the two tx and the transmission problem was still there, I finally end by replacing the long antenna I had and the problem was resolve. The defective antenna is the first one on the photo

I discovered this morning by placing my hand over the defective antenna, the picture was comming back. It gave me the idea to change the channel one to channel four and the problem disappear... it appear only on channel 1 in fact???
Do you understand ?.
p.s. The picture look completly out of tunning like if the receiver was set on a wrong channel, picture black with distord diagonal lines, no picture visible.


I also order this antenna to try on the receiver, what do you think about it, is it good for our application you htink?
http://www.nghobbies...products_id=222

Attached Thumbnails

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Edited by Vrflyer, 03 March 2009 - 07:06 AM.

[Mr.RC-Cam]

The picture look completly out of tunning like if the receiver was set on a wrong channel, picture black with distord diagonal lines, no picture visible.

OK, that symptom helps. What are you using to power the Tx? For example, is it a 3S LiPO, DE's Anyvolt, other DC switcher, or ?

Also, what voltage do you read at the input to the Tx when it is freaked out? Keep in mind that some DMM's will be inaccurate when they come near a RF transmitter, so watch out for that.

I have not tried the HG905RD antenna seen in your last link. I've ignored it because it is long (really long), so its only practical place seems to be on the video Rx. Also, because of its higher gain, its vertical beamwidth is more narrow and less uniform than the HG903RD.

So it might not be a great pick for a model aircraft application. However, from what I have seen in your videos, you fly at very low altitude and close range, so I think it will do fine for you. Honestly, I would prefer the HG903RD on the Rx, but please try the HG905RD and report back on how it compares. Beyond that, I'd recommend a decent patch antenna on the Rx.

[Vrflyer]

I use 3S lipo or a power supply. The voltage of the lipo is 11.36V not plug and drop at 11.26V when the tx is plug, I tried on channel 1 et 4 with same result. I tried also with my power supply (it was adjust at 11.4V coincidently) and also with a freshly charge lipo at 12.6V. I always have the same result: dropof 0.10V and no picture on channel 1 but working fine on channel 4. the picture come back on channel one when I put my hand over the antenna again.


If I power the tx at 12V on channel 1 and drop the voltage,the picture appear normal at littly less than 9V and remain up to 5V, but if I apply 5-8V at power up, I got no picture, the voltage must go at 12V first and reduce to 5-8V to see the picture appear.

[Mr.RC-Cam]

Well, I'm in a bit of daze at the moment. I grabbed a 900Mhz/200mW Tx, same as you have, and bench tested it with a dipole antenna. CH-1 (910MHz) loses PLL lock and goes nuts, just as you reported. I also noticed that CH-2 is unstable and can lose lock too. The other two channels are fine. There seems to be a Tx design problem.

What's really interesting is that I can install my special low pass antenna filter and everything works great. I'm seeing >4dB of signal strength improvement with the compact dipole and >5dB with the standard size dipole (compared to the stock "3dBi" wire whip).

So, there's definitely an advantage to upgrading the antenna. But unfortunately, the 200mW Tx hates "better" antennas when you use CH-1 or CH-2. Given that CH-1 is the only ham-legal North American RF channel, this is a bug that needs some attention. The good news is that the 900MHz 500mW Tx works fine on all the channels.

You know, when it comes to this microwave RF stuff, everything matters. Even the things you least expect!

[Vrflyer]

So if I use the antenna HG903RD on channel 3 or 4, does it will work normally or I still will get some loss in the strengh of the signal.

[Mr.RC-Cam]

I don't have a HG903RD in stock to try (sold out), but I have a Taiwan clone of it. I found that at 1010Mhz and 1040Mhz (CH3/CH4) the gain was about 4dB better than the stock whip. So, that is good.

Now the bad news. One time I powered up on the higher channels and could not get it to work with the dipole. I had to turn off power and try again. So, the problem is still present, but not as bothersome. There is something evil in there that may require an RF exorcism.

You might want to contact your supplier (NG) to see what remedy they have. Perhaps this is all old news to Zoltan and he knows what to do. Please keep us posted on how you solve the problem.

[Vrflyer]

I bought these two 200mw tx from Rangevideo, I will asl anyway to Zoltan if he had heard about a problem with this tx model.

The long antenna that I talk at first is comming from my desk, I got it since a couple of year and I remember I received it from Future hobbies with a very cheap 900tx/ rx. It was not usable I got very poor reception in my house on all channels but was able to see a picture. Perhaps this antenna was not tune for my video tx/rx and is not tune again for my new tx, . It can work on channel 4, 3 etc but too much outune for channel one, is it possible.
What should be the ideal lenght of the antenna (without braiding) for our application, I will measure my antenna.
I don't have any problem on all four channel when I use the new 3dbi or 5dbi antennas received from nghobbies and also with the small antenna received with the tx, I got problem only with the first antenna receive from future hobbies.

[Mr.RC-Cam]

What should be the ideal lenght of the antenna (without braiding) for our application, I will measure my antenna.

It totally depends on the antenna's construction. If it is a classic +2dBi center fed dipole then the elements would be ~3.08 inches at 910Mhz. But, these popular ducky antennas are never built as classic dipoles so their lengths will be different. If you mean the cheap stock wire whip that comes with the Tx, then please see this: Monopole Wire Whip Mod

I don't have any problem on all four channel when I use the new 3dbi or 5dbi antennas received from nghobbies

Which antennas are those? If one of them is the HG903RD 3dBI antenna, then that one did not work for you on CH-1 per your earlier report. But it works now? I am definitely confused. As I mentioned, I have a clone of that dipole on the bench and it caused problems on the 200mW Tx. I shipped my last HG903RD today, so I will have to wait a couple weeks to try the real deal out.

also with the small antenna received with the tx

That one works for me on all channels too, but has low gain performance compared to the dipoles I tried. It is the antenna I used when I reported the gain comparisons in my earlier posts.

[Vrflyer]

Which antennas are those? If one of them is the HG903RD 3dBI antenna, then that one did not work for you on CH-1 per your earlier report. But it works now? I am definitely confused. As I mentioned, I have a clone of that dipole on the bench and it caused problems on the 200mW Tx. I shipped my last HG903RD today, so I will have to wait a couple weeks to try the real deal out.

oups I was not clear. All my video problem happen with a long antenna comming from Future hobbies few years ago, the first one on the photo, not with the HG903RD. I received the HG903RD and the HG905RD from NGhobbies yesterday coincidently. When I try the two new antennas, i don't get problem.
It's why I suspect now the problem is comming from a poor design of the Future hiobbies antennas. I'm surprise you also have problem with your clone antenna.

[Mr.RC-Cam]

I'm surprise you also have problem with your clone antenna.

With the 900MHz/200mW Tx set to CH-1, I easily see the problem with the HG903RD clone and the nice little compact dipole too. The problem goes away when I install my 900MHz low pass antenna filter along with the antennas. Also, I don't see the issue on the 500mW or 1000mW Tx and they work fine with all the antennas I tried.

So, be sure to carefully check your 200mW system out before giving it the final A-Ok. It would be ideal to test it using all four channels, with all expected operating voltages and temperatures. If that all works fine, then you don't have anything to worry about.

[Vrflyer]

I tested the antenna HG905RD (5dbi) from NGhobbies on my receiver while I was using the HG903RD (3dbi) on the plane. I got very bad reception most of my short flight, I landed imemdiatly. Only at take-off I got a decent picture, but at the moment I gain some altitude, I receive near nothing. Clearly this antenna can not be use for my kind of flying, I stay in visual range flying around and above me.

I tried also the 8dbi patch on the receiver and the reception was impressively good. I made only one flight, perhaps I was lucky, but I don't remember such a good reception with no wi-fi for all the flight around me, only when I pass above me I got clear snow in the picture and all around the signal was fading progressively like it should be when the signal begin to be weak, but no other interference visible.

I received today two antenna HG903RD. I hate tripod, I installed my 900mhz receiver in a construction hat and I will use HG903RD with it. I expect to get a decent reception using HG903RD on the tx and also on the rx.

I also ordered one antenna from DPCAV as suggest by Thomas, I want to use it onboard the plane. I will come back to tell you my result.