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alidav

whip antenna cut, the perfect lengh

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I think anyone of you knows this document https://www.lectrosonics.com/phocadownload/ammkit_td.pdf
I am quite confused by the numbers, I need to cut antennas for a Sennheiser 2000 SMA modded in range gw (
558 -626), and want to cut at perfect length corresponding at 590 Mhz, the math tells me that 300/590 is 50.08 cm:4 is 12,7 cm, but on this paper, the block 23 is 4,24" that is 10.7 cm, 2 centimeters less. almost 1 inch.

why?

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Are you sure you printed at the proper DPI? No 'fit to page' or something?
Your math seems correct. (Speed of light / frequency.)

(On closer study, there is a 'test' line on the sheet, that should be 152.4 mm.)

 

But, of course you math defines the antenna lenght, measured from the point where the wire comes out of the shielding.
Now, if the connector is not part of the shielding, that acts as antenna as well. Perhaps that's the difference.

And, if the wire inside the whip is twisted, it could be way longer than the whip length.
How expensive are these things? (Making shorter is easy, longer however..)
Or, run a test, cut it to your length, look up the block where the table says the sweet spot lies, and compare performance.
 

Bouke

 

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As Bouke recommended, check the measurement of the line. It should be 6 inches (152.4 mm). Printers rarely are exactly calibrated; the line is a check on that. You can measure the line and then scale the page to make the line 6 inches. Also, the antenna lengths on the sheet are not 300/MHz. They are what we measured using an impedance bridge with actual antennas on actual transmitters. This will be somewhat different from the theoretical values due to the fact that the transmitter is not a flat infinite ground plane and the SMA connector has the beginning of the wire somewhat above the transmitter. Again, theory and the real world are rarely the same. 

 

When the antenna is on a body, the length is not very critical anyway. If you have it within a plus-minus half inch, you are good to go.

Best Regards,

Larry Fisher

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My computations show the correct antenna length for 590mHz to be 12.7031 cm / 5.0012 in.

 

Of course it takes two things to benefit from that level of accuracy:  a ruler marked in ten-thousandths -- and a perfect world -- both rather hard to find. 

 

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the measurement is correct i printed with adobe illustrato, the line is 6.00 inches long (152.4 mm) and I verified with a ruler, as i told my antenna ( similar to a remote audios) with math calculation is going to be 2 centimeters longer than what lectro paper states. seems that lectrosonics antennas are shorter

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8 minutes ago, alidav said:

the measurement is correct i printed with adobe illustrato, the line is 6.00 inches long (152.4 mm) and I verified with a ruler, as i told my antenna ( similar to a remote audios) with math calculation is going to be 2 centimeters longer than what lectro paper states. seems that lectrosonics antennas are shorter

See post #3

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6 hours ago, LarryF said:

Also, the antenna lengths on the sheet are not 300/MHz. They are what we measured using an impedance bridge with actual antennas on actual transmitters. This will be somewhat different from the theoretical values due to the fact that the transmitter is not a flat infinite ground plane and the SMA connector has the beginning of the wire somewhat above the transmitter. 

 

When the antenna is on a body, the length is not very critical anyway. If you have it within a plus-minus half inch, you are good to go.

Best Regards,

Larry Fisher

 

 

this applies for all the devices or just for lectros?  (i ve never seen the theoretical values respected on other brand also).

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2 hours ago, alidav said:

this applies for all the devices or just for lectros?  (i ve never seen the theoretical values respected on other brand also).

The lengths are only measured for the Lectros but the SMA connectors are roughly the same sizes and all have about the same spacing from the transmitter body surface to the start of the antenna wire. The SMA connector is not a part of the radiator as its shell is grounded to the case. All the transmitters have roughly the same shape so I would say the Lectro lengths are good to "copy" for other transmitters also.

Best Regards,

Larry Fisher

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