Strangest thing with my antenna!

Discuss commercial and home made antennas.
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Donniesito
Posts: 2
Joined: Mon Jun 17, 2019 10:49 pm

Strangest thing with my antenna!

Post by Donniesito » Mon Jun 17, 2019 11:02 pm

As a quick note: I have an aerial in my attic for TV. (We get a lot of digital stations; no need to pay a high cable bill.) I also live in a VERY small town in upstate New York that's closer to Canada than Pennsylvania. Just to give you an idea where I am geographically.

Anyway, the coax from the antenna in the attic is fed to an RF Amplifier and splitter. Out of curiosity I connected the coax to my SDR and got poor results -- only picking up the most powerful stations. But I didn't get into SDR to listen to commercial radio broadcasts! Well until I build or purchase an antenna specifically for my SDR, I'm using the "tv antenna" in my attic.

Here's what happened: I had started SDR# and was in the process of hooking up the coax, when suddenly a ton of stations popped up on the waterfall! The odd thing is that I'd already had SDR# running and noticed I hadn't put the coax in all the way. Only the center conductor was connected. I stopped and left it as is & ran through the bands.. I got a LOT more than I ever had before.

My theory is that since I was only using the center conductor, I basically used the 30 or so feet of vertical coax as an antenna (bypassing the TV antenna entirely.)

What do you all think? Is my theory correct? Antenna theory is WAY above my head, but my theory makes sense to me ;-)

rtlsdrblog
Site Admin
Posts: 2280
Joined: Mon Nov 19, 2012 11:54 pm

Re: Strangest thing with my antenna!

Post by rtlsdrblog » Tue Jun 18, 2019 1:10 am

Yes that could be happening. The inner conductor should actually be shielded, so if you can somehow connect the outer shield of the coax cable to your SDR if might work even better as that is more exposed to the RF. Really you want a proper antenna like a Discone or dipole up in your attic though.

The TV antenna itself might not be so great. First there is an amplifier in line, and I assume that takes bias tee power from a unit plugged into the TV. Without that unit the amplifier would be unpowered. Secondary, TV antennas are usually Yagi's, which are directional antennas. So it will only receive well in the direction that it's pointing.

HighSNR
Posts: 43
Joined: Mon Sep 18, 2017 8:14 am

Re: Strangest thing with my antenna!

Post by HighSNR » Tue Jun 18, 2019 2:40 am

That sounds like the most likely scenario.

As admin says, the yagi is unidirectional, but it is also tuned to the TV frequencies and may not perform well outside this band.

Additionally the amplifier may also contain a filter to reject out of band signals.

Some amplifiers have a second input for an FM antenna(80 to 110MHz). This is normally used to connect hi-fi radio receivers. But again, this may be filtered to block out other signals.

Donniesito
Posts: 2
Joined: Mon Jun 17, 2019 10:49 pm

Re: Strangest thing with my antenna!

Post by Donniesito » Tue Jun 18, 2019 2:20 pm

Yep - I'm planning on experimenting with building my own. If the experiments don't go well, I'll buy one.

BTW, the antenna amp is powered from a power supply.

Yeah - I know it's not the best scenario, but it won't be long until I've built/bought another antenna ;-)

qrp
Posts: 23
Joined: Wed May 22, 2019 11:16 pm

Re: Strangest thing with my antenna!

Post by qrp » Wed Jul 10, 2019 8:45 am

Donniesito wrote:
Mon Jun 17, 2019 11:02 pm
My theory is that since I was only using the center conductor, I basically used the 30 or so feet of vertical coax as an antenna (bypassing the TV antenna entirely.)

What do you all think? Is my theory correct?
No, your theory is wrong.

The problem here is that when you connect your antenna completely, it has too high signal level, so your receiver is overloaded and cannot receive weak stations. It cannot see it due to insufficient dynamic range. The same as you cannot see the dark street in detail, when someone directed bright flashlight beam into your eyes. :)

When you connect your antenna partially, it works like attenuator. So, your receiver is not overloaded and can see weak signals.

You can adjust good signal level with by inserting attenuator between antenna and your receiver. You're need to choose proper attenuator value. Because too high value will leads to weak reception, but too small value will leads to ADC overload.

But first just make sure that Gain settings is reduced to zero and RTL AGC and Tuner AGC are disabled. It may be the reason of ADC overload.

You can find attenuators on aliexpress or ebay. For example: https://www.aliexpress.com/item/32920088808.html

I recommend you to buy three values: 10 dB, 20 dB and 40 dB. 40 dB value will be used very rare, so if it cost much for you, you can buy just 10 dB and 20 dB. In most cases it will be enough for you. You can combine them together, for example 10 dB in series with 20 dB = 30 dB.

By experimenting with different attenuator values, you can adjust input signal to the level which will not overload your receiver. Different antennas and different receiver will require different value, so you're needs for experimenting which value is better for your case.

You can monitor when input signal is too high for your receiver in HDSDR. You can find this information at right top corner of spectrum area. It shows as "-25 dBFS". This dBFS value means how much input signal is below maximum value for your ADC. When it come 0 dBFS it means that the maximum level on the input is reached. The good level for receiver is about -30 dBFS. The value -10 is acceptable, but it may leads to some distortions. The value higher than -10 dBFS is not recommended.

HDSDR shows that value in red when your ADC is overloaded, in yellow when input signal level is near to overload value and in green when input signal level is normal. So, when it shows red or yellow, just reduce gain in the settings and if it doesn't help, then insert attenuator between antenna and receiver.

I would recommend you to disable RTL AGC and Tuner AGC when learning it. Because these settings changes gain dynamically and it may confuse you.

There is also much better way to fix receiver overload issue by adding good bandpass filter between antenna and receiver. The filter will attenuate all frequencies except the narrow band which you're need to listen. But they are expensive.

If you want to try filters, you can start by experimenting with FM reject filter. It attenuates 88-108 band.
https://www.rtl-sdr.com/rtl-sdr-com-bro ... -for-sale/

If you have too strong FM stations in your location it can help for a little.
Then, if you needs better filter, you can look at bandbass filters for the frequency band which you're interesting for.
For example:
https://ww2.minicircuits.com/WebStore/RF-Filters.html

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