Antenna mystery : UHF/VHF antenna better than longwire ?!

Useful information regarding antennas for SDR products.
sebastien
Posts: 2
Joined: Sun Sep 04, 2016 12:57 pm

Antenna mystery : UHF/VHF antenna better than longwire ?!

Postby sebastien » Sun Sep 04, 2016 1:13 pm

Hi !

First, let me know that I love my SDRplay, it is a great receiver that compete with much higher hardware receivers that are more expensive :)

That said I am confronted to a mystery I cannot explain : on HF, I obtain way better reception with an indoor bi-band 144/430 magmount antenna, 35 centimeters long) placed on the window than with an indoor 5-6 meters long wire coupled with a 9:1 unun (I live in a city and cannot have big outdoor antenna).

I made some test with the long wire thrown outside the window, and the reception was not better, whatever the gain and LNA settings in SDRuno. The best results so far are always with my biband antenna, LNA on, and -22dB of gain reduction. How can this be ?

My biband antenna : http://www.sebastien-billard.fr/tactico ... 44-430.jpg

Another question is : how can I improve my reception with an indoor antenna ? The best placement for my antenna I can use is my window.
Last edited by sebastien on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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JMG
Posts: 17
Joined: Thu Mar 26, 2015 9:58 pm

Re: Antenna mystery : UHF/VHF antenna better than longwire ?!

Postby JMG » Wed Sep 07, 2016 10:47 am

No myth....
At HF a well matched antenna will receive a noise level which is up to 50dB (!) above the noise level of the receiver. This may load the SDR receiver (having a limited daynamic range) to its limits.
Less is more!
The antenna will be good enough if the noise floor of the receiver will rise by more than 8dB when connecting the antenna to the receiver. Any better antenna will not improve the S/N ratio any further.
The antenna should be placed at a low noise position.
A well placed VHF/UHF discone antenna gives surprisingly good results over the total frequency range of 50 kHz to 2 GHz.
Last edited by JMG on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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NK7Z
Posts: 9
Joined: Fri Sep 02, 2016 1:14 pm

Re: Antenna mystery : UHF/VHF antenna better than longwire ?!

Postby NK7Z » Wed Sep 07, 2016 7:33 pm

Kill the AGC, turn down the gain, and kill the LNA. I am getting really good results using a 90 foot TTFD as the recieve antenna and an SDRPlay.

See:
http://nk7z.net/sdr-for-rfi-survey-p1/
for more on this...
Last edited by NK7Z on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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sebastien
Posts: 2
Joined: Sun Sep 04, 2016 12:57 pm

Re: Antenna mystery : UHF/VHF antenna better than longwire ?!

Postby sebastien » Wed Sep 07, 2016 9:46 pm

@ NK7Z : I live in an urban setting and cannot put big antennas outside. With a small passive indoor antenna why kill the LNA (I don't use AGC) ?

@ JMG : if I understand well, what you mean is that SDRs such as SDRplay behave more like a consumer radio than a hardware receiver and are way too sensitive for big antennas ? (I have limited knowledge in antennas)
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NK7Z
Posts: 9
Joined: Fri Sep 02, 2016 1:14 pm

Re: Antenna mystery : UHF/VHF antenna better than longwire ?!

Postby NK7Z » Sat Sep 10, 2016 10:30 pm

Sebastien,
Clearly you would not want to kill the LNA if you are using a smaller window mounted antenna shown in your link. I assumed you were using a long wire as you stated... Your response left off the long wire you mentioned, so you probably would want to keep the LNA on in that case. My answer was relating to your long wire comment only.

Given that you are using what looks to be a 2/70cm antenna on the window, it is not surprising that your HF performance is so poor... Try your idea of a long wire, long being defined as something like 50 or so feet of wire. You may then find, that with that length of wire, you need to kill the LNA due to overload.

Dave
http://www.nk7z.net
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