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Radio Mic Receiver Antenna polarisation in diversity setup.


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Hey...

 

I've been thinking that in a diversity setup for radio mic receivers, using the sharkfin style antennas, maybe having one vertical and one horizontal would make better use of the diversity feature. But most people seem to have 2 in vertical polarisation pointing in the same general direction, therefore only the distance between the two is giving any advantage for the diversity.

I have seen the occasional photo with one vertical, one horizontal, but very rarely.

 

I will try a test in the next week if i get the opportunity, but i would be interested in hearing peoples comments.

 

Cheers,

Nick

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Thanks for the comments guys,

I haven't seen Helicals on set, but can see that a circular polarised antenna would be good, I actually use them for another application as video send and receiver antennas.

 

"It all depends"...

thats true, maybe a direct question if i may.

Do you have a diversity receiver with two sharkfins? (Very common here)

If so, do you have them both mounted vertically? (Again very common setup that i have seen)

Would you consider mounting one horizontally?

If not, why?

 

The way i am looking at it, one vertical and one horizontal will make a better use of the diversity feature.

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Thanks for the comments guys,

I haven't seen Helicals on set, but can see that a circular polarised antenna would be good, I actually use them for another application as video send and receiver antennas.

 

"It all depends"...

thats true, maybe a direct question if i may.

Do you have a diversity receiver with two sharkfins? (Very common here)

If so, do you have them both mounted vertically? (Again very common setup that i have seen)

Would you consider mounting one horizontally?

If not, why?

 

The way i am looking at it, one vertical and one horizontal will make a better use of the diversity feature.

Our repeated walk tests don't show that it works as well as you might think. It's not bad or wrong but doesn't give any consistent improvement. If the the transmitter antenna is vertical, one can argue that having both antennas vertical, you will give you a higher probability of intercepting the maximum signal. YMMV

Best Regards,

Larry F

Lectro

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Thanks Larry and John,

The black art of RF, all very interesting stuff.

I think I'll make up a bracket so I can rotate the receiver antenna for the occasions when the transmitter is horizontal (car travelling the transmitter offer ends up tucked in the visor)

cheers,

nick

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" Do you have a diversity receiver with two sharkfins? (Very common here)

If so, do you have them both mounted vertically? (Again very common setup that i have seen)

Would you consider mounting one horizontally? If not, why? "

yes, yes, yes.

 

" one vertical and one horizontal will make a better use of the diversity feature. "

maybe

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receiving video with diversity ants in the UHF range it is obvious right away that circular polarized antennas (helix)  have less flicker. two helix are much better than one and three are again better if you have a 3 antenna tuner. In video we have 4 antenna tuners for good reason. Tx with helix and 2 (or more)  helix to Rx are best. How to get a CP on a hidden Tx is a huge question though. Noone has made a one turn Helix ? why not? also the 6-8 turn RX Helixes have much better gain than the "sharkfins" and are therefor useful for  audio only FM reception and transmission.

digital transmission ( whatever modulation or encoding used) thrives on diversity. Our walk test experiments though did not show any improvements with Helixes, providing the gain is similar.  - I know weird.

anyway what's imperative for video RX is not that noticeable for audio.

there is lots more on this on my literature pages on my site 

www.wolfvid.com

and a must read of course is the paper on RF on the Lectro site.

I have written some detailed manuals on antenna use and give them out to all video guys with the note that if they find the sercret codeword I give them a 100$. Noone has called yet. I guess a precondition for moviework is illiteracy?

we also sell a cheap ugly green "sharkfin" that's excellent for 400-1000 MHz. the other good antenna like this is Lectros wich is sturdier and more expensive. there are some popular others out there that suffer from mid-frequency suck out (the technical term) that are so-so. then there are colorful ones from NJ that I have not tested. spacing of div ants is critical.

Of course keep cables to Rx as short as possible.

this is not a black art, people who say that are covering up their ignorance.

 

wolf (I don't know if i should snicker or sniffle)

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Larry, please correct me if I'm wrong here...

 

With the Ratio Diversity system used by most portable Lectrosonics systems (which are the most widely used systems in North America for film and video production), The polarization of the receiver antennas doesn't matter, on average. This is because the ratio diversity system is always combining both antennas instead of always switching to only the antenna with the best signal. Therefor, if there is a 90 degree difference between antennas, an advantage of one polarity will be always offset by the disadvantage of the other. If both antennas have the same polarity, then there is at least a chance of both occasionally being in the best position at the same time, which, when combined, would produce a better signal than could ever occur in two antennas with a 90 degree offset.

 

Receivers with antenna-diversity systems (I dislike the misnomer "true diversity") switch to only the antenna with the best signal, so a 90 degree offset between antennas could be beneficial.

 

Glen Trew

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Hi Glen,

Ratio diversity combines the audio signals not the RF signals. Weighting is given to the better audio signal so that the signal to noise ratio is improved when the signals have close to the same SNR. If one signal has a SNR  2 to 3 dB worse than the better signal, then it is discarded. Ratioing or blending the signals rather than gating between the two keeps the best possible SNR. If you have good RF signals at both antennas, which is 99% of the time, then ratioing will give you a 3 dB improvement over gating.

 

Receivers with antenna-diversity systems like ours, when in antenna diversity mode, that combine the antennas in or out of phase,  do not seem to benefit from angled antennas (one horizontal and one vertical). That is according to walk tests that we have done in our own "zone of death" (rows of parked cars in the Lectro lot). Others on this forum have found differences in performance with a small advantage to tilting the antennas in different directions. All I can definitively say is YMMV and "It Depends". I can safely say the improvements are small if they exist. On the other hand, small improvements are sometimes all you need. (Isn't it nice to have some one come down solidly in the middle and firmly on both sides of the question.)

 

Speaking of tilting antennas, the SMJ hinged SMA antenna will be back in stock shortly for those that want to angle antennas on body packs or the SRb. The new antenna is at least 5 times stronger than the previous one.

Best Regards
Larry Fisher
Lectrosonics,

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