Comet vs. Diamond gain specs, dB vs. dBi?

[sparklehorse]

Hello,
I’m looking to replace my Diamond X50A antenna with something better for VHF/UHF public safety (monitoring only). I’ve been leaning toward the Diamond X300A, but like the X50 I already have, the X300 is tuned for the 2m/440 Ham bands. Then today I learned of the Comet GP-6NC, which seems to be a nearly identical design to the X300, but is tuned to the Public Safety bands. Both the GP6 and X300 are a co-linear design, each about 10 ft tall, each with phasing of 2- 5/8λ, and 5- 5/8λ.

Trouble is, Comet lists the gain for their GP6-NC as 6.5/9.0 in dBi:

VHF/UHF Dual-Band | Comet Antenna

Whereas Diamond lists the gain for their X300A also as 6.5/9.0, but in dB:

https://www.diamondantenna.net/x300a.html

If you compare other Comet models vs. similar Diamond antennas you’ll see the same issue with respect to dB vs. dBi gain figures. For instance, the Diamond X50A gain is listed as 4.5/7.2 in dB:

https://www.diamondantenna.net/x50a.html

Whereas the Comet GP-3 gain is also 4.5/7.2, but it’s listed in dBi:

Dual-Band | Comet Antenna

Again, these are seemingly identical antennas.

The real world difference of dB vs. dBi is 2.15 dB, which is not insignificant. Meaning if these specs are true, the Diamond models all have a 2.15 dB advantage over their Comet counterparts. But is that actually the case? I’m guessing these antennas, the GP6 vs X300, or the GP3 vs. X50, are nearly identical in real world performance, but one company is simply using a more accurate gain metric than the other. Which is surprising to me as I was under the impression that both were good antenna makers.

Am I missing something here? Is the Comet GP6 virtually identical to the Diamond X300 in spite of the 2.15 dB difference in listed specs? Or is the X300 actually better?

Thanks for your help!!

.

 

[AK9R]

If the specs don't say what the dB is relative to, e.g. dBi or dBd, then you are just guessing.

My guess is that the Diamond specs are actually in dBi if they don't explicitly say so. The fact that the lengths are similar is the tip-off. Since you rarely get something for nothing, if the lengths are similar, they gain is probably similar.

 

[prcguy]

In my experience owning many Comet base antennas and comparing to commercial versions, the gain is less than advertised in dBi.

Something to consider when comparing a similar Comet and Diamond base antenna is the size. They will be very close in actual gain when the lengths are about the same. If a Comet and Diamond 2m antenna are the same length but the advertising says one has several more dB gain, assume they are about the same gain.

 

[nd5y]

There is no way a 10 foot long 146 MHz antenna can have 6.5 dBd gain.

 

[Project25_MASTR]

Oh gain. Unless it is specifically listed as dBd…always assume dBi when dealing with the amateur radio antenna companies. 0 dB ≠ 2.15 dBi. 0 dBd = 2.15 dBi.

The reality of it though, the performance difference of an antenna specifically tuned to the commercial bands of interest for monitor only purposes versus one tuned to the nearby amateur bands isn't going to be noticeable.

 

[RFI-EMI-GUY]

Commercial LMR antennas are usually listed in dBd ( reference to dipole) while ham antennas are sometimes rated in dBi ( reference to isotropic). It is about marketing, a dBi antenna always specs 2.15 dB greater than same antenna measured in dBd. So buyers are attracted to buying a 6.15 dBi antenna vs a 4 dBd antenna. There are 0 dBd reference dipole antenna designs which are published by NIST. testing against one of these references can yield a dBd value. Adding the theoretical 2.15 dB attains a dBi value.

Commercial microwave antennas are rated in dBi because they are usually modeled from an theoretical isotropic reference point.

If a manufacturer says only "dB" they are ignorant or trying to hide something.



Sent from my SM-T350 using Tapatalk

 

[sparklehorse]

Thank you all for the very helpful information. I had always assumed dB and dBd were the same, now I know to assume it means dBi until proven otherwise. Thank you! Learn something new every day. I went ahead and ordered the Comet GP-6NC, which I think I’ll be quite happy with once I figure out the mysterious N connector issue that I’ve created a new thread to address.
Thanks again!

 

[AK9R]

Assume nothing. If the antenna manufacturer quotes gain in just "dB", you will have to ask them what that means.