Poor performance with extended aerial

Mar 08, 2007 5 Replies

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gnewsaccount 19 years ago

I have an 868MHz design incorporating a reference design for the transceiver. The aerial is connected to a PCB mounted SMA connector. The board sits in a metal box and the aerial passes partly through the box to attach to the board.

Range is acceptable if not great. The aerial manufacturer retuned the aerial for our box and said that unfortunately the most sensitive part of the aerial was inside the box thus we were losing range.

We tried removing the sma connector from the board and attaching a short 3-4 inches of coax with a bulk head mounting SMA connector to original mounting holes of the pcb mount connector. range was reduce to about 4 feet. We tried in and out of the box with no success. On looking at the IQ signals it appears that the Q signal is largely unaffected by the change, may be slighly higher amplitude but the I signal goes from good to collapsing over the 4 feet distance. The cable with connector is off the shelf and we hae tried several cables and several boards. Any clues what may be happening by adding the additional cable.

Thanks for any help

Paul

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Genome 19 years ago

In simple terms, having extended your aerial you have to play with the tuning dial(s) and other parts to optimise response.

Trust me, I'm not an expert.

DNA

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Fred Bloggs 19 years ago

The reference design most likely is tuning out the "aerial" reactance over a narrow frequency band. You have perturbed this reactance with the impedance transformation resulting from the section of coax extension line. Retune the xmit reactance element, if at all possible, for best results.

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mpm 19 years ago

If you want to do this the right way:

Rent (or find someone with) an HP8753C Vector Network Analyzer (or equiv). If you are unfamiliar with how to set one up for the measurement, you'll need a qualified operator. Perform an S11 parameter test and locate (tune, or adjust) the antenna/ feedline for best Return Loss. That will give you your best throughput.

Next, you can deal with directional pattern problems that may be introduced by your enclosure or near-field antenna obstructions or other parasitic elements, if any.

-mpm

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gnewsaccount 19 years ago

The transceiver has an LC network to splitting the incomming signal into a refA refB signal are these the components I need to alter or is it best done at the aerial.

Also If I tune everything up, If I then want to enable the use of an extension aerial are we saying it will be no good just having a length of cable plus say an 868 Yagi aerial as I would probably get more loss than just that of the cable and connectors, but that I would also have to either tune the aerial for the length of cable fitted or provide a tuning components selectable on what aerial/cable is fitted. We didn't seem to need to do this on the old 173MHz system.

Paul

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gnewsaccount 19 years ago

Another thing, I'm not just seeing poor performance -6db -12db reduction I'm seeing terrible performance -30db.

Why does it only affect Iamp shouldn't it affect both as the same signal entering the tranceiver is mixed with the in and quad phase local oscillators.

Another thing actually I tried using a SMA to SMA to original aerial no coax involved and the same result. Now as I removed each section, first the aerial and then the SMA-SMA connector the signal first moved from being only on the Qamp to only being on the Iamp to only being on the Qamp again.

I don't understand.

Paul

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