RF transmitter problem

[nishal]

Hi all,
I tried to send a data using RF transmitter (Part No ST-TR1100+TR from www.summitek.com.tw) fitted with telescopic antenna from a 60 ft high which is a metal enclosed room to the ground point. Transmission fails. Where as transmission from ground to metal room works pretty well.

What could be the reason for failure in transmission? is there solution?

I am using 433.92 Mhz, 1.2 kbps data rate, 2 FSK Modulation.

Thanks in advance.

Nishal

 

[GVVIN]

could you elaborate more.

Is the pair working properly at ground at small distance. as well as 60 feet on open space (not in metal room).
Are you using same antenna , Tx power etc at both ends.

 

[nishal]

Thanks gvvin. The pair working properly at ground within a distance of 250 to 300 mtrs (non line of sight). I am using same antenna on both units. Though using same modules on both units, one is assigned to Rx while the other one to work as Tx. I haven't tried from 60 ft high on open space other than metalic room . However, reception in the metallic room using Rx unit works well. My problem is Tx unit from metallic room of 60 ft high doesnt work.


Nishal

 

[GVVIN]

Hi Nishal,

Sounds intresting.
My views....
I don't know whether a telescopic antenna is suitable for this application.
Since you are using same antenna for Tx and Rx and you are able to receive properly in metal room , you don't have a problem with your antenna there.

So may be you have to look into the TX part...may be your power supply in metal room is not able to give sufficient DC to your amp . ..
or there is some thing that is different up there apart from the metal obstruction....
also try line of sight from metal room. (thru window or door)

Regards
GV

 

[FvM]

I think you're aware that talking of a a metal enclosed room doesn't provide more information than that radio transmission is disturbed to an unknown extent. It can't be expected to work reliable under this conditions generally.

In detail, the metal enclosed room probably has a window or slits to pass the RF, but the signal attenuation may be severe. If you simply assume, that you're operating near or above the usable transmission distance under this conditions, there is no need to explain exactly, why one direction fails. The other probably works only by chance.

Additionally, existence of in-band interferences may be a sufficient explanation if you need one. If you have a receiver with an RSSI output or a spectrum analyzer, you may check this assumption.

 

[nishal]

Thanks GVVIN, FvM. I tried dipole antenna and switched on to telescopic just because dipole is directional but it works. Is there any suggestion/advice in selecting appropriate antenna? Eager to know any correction method to achieve transmission result in mettalic enclosed room.

Nishal

 

[vfone]

Looks that your transmitter behaves different for the two environments. Most probably the metal enclosure detune the antenna (or even the output match, if is too close), reducing in this way the radiated power.
On the other way the telescopic antenna, as any other λ/4 monopole antenna, works better when use a perfect ground plane. For imperfect ground-planes, the direction of peak directivity are at an angle above the horizon, so the pattern of the same telescopic antenna will be different for the two locations.
Try to change your telescopic antenna with a Helix antenna in axial mode. This antenna needs a ground-plane, but is not so sensitive to ground variations as a λ/4 monopole.

 

[RF-OM]

To nishal,
I think your case is not so strange or rare as it may looks. This happens often in RF and microwave engineering. All text books stated that transmitting and receiving properties of antenna and radio path are equal, but more often than not it is not true. The metal room constructed from metal sheets and probably they have not so good electrical connections and long seams. There may be some non-uniform electrical current distribution across these sheets that may be a reason of electromagnetic field creation inside the room. This is one of the major problems with screen rooms. There also may be so-called re-radiation. It is when metal parts have poor contact and poor contact is non-linear element. One signal can modulate another (sometimes main power line current) and on the shoulders of this more powerful signal go inside the room. Often in this case there is strong 50 or 60 Hz hum that disappears only if you disconnect antenna. This is for receive path. For transmit path you have totally different picture and this is why these two paths are different. Of course, I cannot be absolutely sure that you have exact the same case as I described, but it is very possible.