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Is it normal for an antenna not have lowest swr in its advertised band?

Posted: Sun Feb 25, 2018 4:34 am
by Github
I bought this small CB antenna, it is advertised to work at 477 MHz. But looks like the swr is not the lowest at 477 mhz. Is it normal?

Re: Is it normal for an antenna not have lowest swr in its advertised band?

Posted: Sun Feb 25, 2018 9:40 am
by rtlsdrblog
How are you doing the scans? Remember that VSWR measurements with a noise source, directional coupler and RTL-SDR are not terribly accurate. I do see a dip at 477 MHz though. It is possible that antennas will have multiple resonant points. Remember that just because an antenna is resonant doesn't necessarily mean that it will receive well at that frequency. Depends on the radiation pattern too.

Re: Is it normal for an antenna not have lowest swr in its advertised band?

Posted: Mon Feb 26, 2018 2:44 am
by Aussie Susan
The scale you are using is quite large.
I guess the real question is, does it have the SWR that the manufacturer stated at the frequency of interest? If it does then you have what you bought. If not then you might have a case for misleading advertising and getting your money back (certainly here in Australian and there may be other penalties available but legal systems vary all around the world).
The trace, certainly above 800MHz, looks to me as though you are actually measuring the SWR of the cable mostly and not the antenna.
Susan

Re: Is it normal for an antenna not have lowest swr in its advertised band?

Posted: Mon Feb 26, 2018 3:23 am
by Github
Aussie Susan wrote:
Mon Feb 26, 2018 2:44 am
The scale you are using is quite large.
I guess the real question is, does it have the SWR that the manufacturer stated at the frequency of interest? If it does then you have what you bought. If not then you might have a case for misleading advertising and getting your money back (certainly here in Australian and there may be other penalties available but legal systems vary all around the world).
The trace, certainly above 800MHz, looks to me as though you are actually measuring the SWR of the cable mostly and not the antenna.
Susan
I am using a rf bridge and dummy load from ebay to test the swr. I don't know how to get the exact swr value at a certain frequency. Do you have any link about testing actual swr? Thanks.

Re: Is it normal for an antenna not have lowest swr in its advertised band?

Posted: Mon Feb 26, 2018 3:25 am
by Github
rtlsdrblog wrote:
Sun Feb 25, 2018 9:40 am
How are you doing the scans? Remember that VSWR measurements with a noise source, directional coupler and RTL-SDR are not terribly accurate. I do see a dip at 477 MHz though. It is possible that antennas will have multiple resonant points. Remember that just because an antenna is resonant doesn't necessarily mean that it will receive well at that frequency. Depends on the radiation pattern too.
I did the scan with rf bridge, I believe it is more accurate compare to directional coupler. I don't know how to get the exact swr reading by using rtl-sdr. Do you have any article about this? Thanks.

Re: Is it normal for an antenna not have lowest swr in its advertised band?

Posted: Mon Feb 26, 2018 6:46 am
by AcidJazz
First of all, gain 49.6 is too much, I think. You need to try change gain for the maximum dynamic range.

Re: Is it normal for an antenna not have lowest swr in its advertised band?

Posted: Mon Feb 26, 2018 8:47 am
by Github
AcidJazz wrote:
Mon Feb 26, 2018 6:46 am
First of all, gain 49.6 is too much, I think. You need to try change gain for the maximum dynamic range.
If I set the gain too low, I can't see any obvious dip on the curve.

Re: Is it normal for an antenna not have lowest swr in its advertised band?

Posted: Mon Feb 26, 2018 8:06 pm
by AcidJazz
Simple test, put to DUT port piece of wire and you should get something like this
2018-02-26_22-33-24.jpg
2018-02-26_22-33-24.jpg (55.63 KiB) Viewed 4740 times
It's wire 48 cm length. I use bg7tbl noise source & china swr bridge (3 ports, 50Ohm load is inside). Red line is "zero". My optimum gain set is 2.7, but it depends of noise power from the generator.