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Sennheiser G3 SMA antenna mod. Cut length?


DrDranim
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It seems every walkthrough of how to do this mod ends with "cut your antenna length according to the Lectro guide" or "cut to the guide which came with your antenna."
These antennas are from AliExpress, which I've seen recommended here and in other SMA mod guides and they don't come with a cut guide.
My antenna basics question being, since the Gx antennas have no external metal shielding (rubber coated only), does the metal of the pack effectively become the shield? Does that determine the "start point" of length of the antenna?

I know there's a certain allowable tolerance for how exacting you need to be with the length of the whips for these kits, but I'm just one of those folks who'd like to be exact as I can.

TIA!

G3_Mod_Antenna-Length.jpg

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"...since the Gx antennas have no external metal shielding (rubber coated only), does the metal of the pack effectively become the shield? Does that determine the "start point" of length of the antenna?"

The simple answer is yes. Also, once the antenna is near or touching a warm bag of salt water (also known as talent),  all length considerations are out the door. The antenna length is only correct for a transmitter in free space, a rare occurence.

Best Regards,

Lawrence Fisher

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Thank you, Larry.

 

A follow-up Q:
The stock length of the Senn A-band (516-588MHz) whip, as measured from the body pack to the end of the steel wire is ~125mm.
From the limited amount of reading I've done on 1/4-wave whip length, using the equation 300/CenterFreq/4 results that the 1/4-wave length of the A-band's center frequency (537MHz) should be ~139.5mm.
I referenced Lectrosonic's whip cutting guide and they recommend a whip length of ~120.4mm for Block 21 (center freq 550.35MHz). However, using the (300/CenterFreq/4) equation, the resulting 1/4-wave length is ~136.27mm.

 

Any idea why the given/recommended whip lengths are shorter than what that equation gives? (I'm sure it has something to do with theory beyond my tiny brain!)

(And just because it's interesting, here's a pic of the Senn antenn stripped of insulation.)

naked_whip.jpg

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The theoretical 1/4 wave length is calculated using an infinite ground plane with the antenna feedpoint just above that plane. With a body pack transmitter and a wire soldered to the PC board (original G3) the feedpoint is at the ground plane but the ground plane itself (transmitter case) is nowhere equivalent. When you add on an SMA connector, the feed point of the wire is raised away from the ground plane further changing the parameters. Rather than spending a week making models and simulations that probably would be wrong anyway, we hooked up a network analyser and just measured the best match. That is where our cutting chart comes from. You will note that the recommended lengths are not strictly proportional to the frequencies, i.e., twice the frequency is not half the length of wire. Theory and the real world are usually different.

Again, getting the perfect length is a vanishingly small consideration when the transmitter mounted on a person. And a microscopic factor when a camera link is on your frequency.

Best Regards,

Lawrence Fisher

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Extremely insightful. Thank you again, Larry.

Interestingly, Sennheiser's stock whip length for their A band (516-588MHz) is exactly the same length as they manufacture for their A1 band (470-516MHz). Guess they really stand by the length not mattering as soon as you put the pack on a "warm bag of saltwater" 😆.

 

I ended up cutting my A band to stock length (~125mm) and A1 band to ~136mm.

 

I also discovered that on the G3 Tx units, the whip is electrically connected to the body/chasis. There's no isolation like there is with the Rx units. So 🤷‍♂️, just strange. Everything is working post-mod, though. Success?

SennPacks.jpg

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  • 2 months later...
On 2/21/2022 at 1:58 PM, DrDranim said:

I also discovered that on the G3 Tx units, the whip is electrically connected to the body/chasis. There's no isolation like there is with the Rx units. So 🤷‍♂️, just strange. Everything is working post-mod, though. Success?

 

I'm not clear what you're describing here. Aren't the whips on the G3 Tx units the same as on the Rx units? I've not seen this. Can you post a pic of how the whip is connected to the chassis?

 

I'm making (and selling) SMA adapters for the Sennheiser bodypacks, and haven't seen this, but obviously would want to make sure what I'm making isn't incompatible with those units.

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3 hours ago, TimTheFoolMan said:

[snip] Can you post a pic of how the whip is connected to the chassis? [snip]

The final element in the RF filter at the RF output may be an inductor to ground (chassis) or an LC resonator to ground. Either case would measure zero Ohms to ground or chassis. Since it seems to work well, I wouldn't worry about it.

Best Regards,

Lawrence Fisher

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