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Question about antenna frequency response or fidelity

M

MediaMan

I am in management with a cursory knowledge of engineering matters. Have a situation where I need to decide between a Shively 6813 antenna or because of weight limits, consider using a Shively 6812. I understand the differences in weight and one requires pressurization and one doesn't. While I have a translator and a full power FM to build out, the full power ERP is only 1 kW. Translator will be 250 watts on same tower but different antenna. Is there any difference in the audio quality produced by the 6812 and the 6813...I have always referred to it as frequency response or bandwith. Is there an engineer here who could interpret the specs of each antenna and post response.

Here is link to 6812:

www.shively.com

6812 Antenna Series - Shively Labs

Low Power - Circularly Polarized
www.shively.com www.shively.com

Here is link to 6813:

www.shively.com

6813 - Shively Labs

Medium Power - Circulary Polarized
www.shively.com www.shively.com
 
There is no difference whatsoever in the audio quality. There are issues with both of the antennas as a result of their narrow bandwidth. They can detune is time if the fibreglass insulator degrades. You can see that as the white part in he 6812 photo. The 6813 has a similar part. That can be a problem if you are somewhere with high atmospheric pollution. The antenna will also detune if it gets iced. The radome will help with that. The narrow bandwidth of the antenna can be a problem for some transmitters, causing instability. Don't try to use a BW transmitter with one of these. None of the issues affect the audio quality though. They all cause high reflected power/bad VSWR.

I would choose the 6813 as it will be slightly less narrow than the 6812 which will reduce the problems above. You do not have to pressurise it.
 
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It’s disappointing that Shively doesn’t publish full RF measurement data. In FM, bandwidth of the RF system first affects stereo separation and distortion. Reduced bandwidth can also affect or aggravate multipath. It would be sort of surprising that an antenna these days could be bad enough to impact any of that, but again, I couldn’t find measurements.
 
This is a sweep of a single 6812 element that is tested several years back. The bandwidth at 20dB return loss (VSWR 1.2:1) is more than 300kHz. More thane enough for a stereo signal. I would expect the 6813 to be a little wider. It is true that a narrowband antenna or filter can affect stereo separation. And yes the lack of data from antenna manufacturers is frustrating.

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Audio frequency response of the program is not affeced so long as the frequency response of the antenna is flat or almost flat over the width of the channel being broadcast on or received. For example, for AM broadcast radio, the channel width is about 10 KHz. If within a particular 10 KHz (and a little more on each side to account for receivers with a slightly wider channel bandwidth) the frequency response have a peak or valley, then the program will be distorted.
 


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