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Author Topic: Cross Modulation Problem with Active Antenna?  (Read 767 times)

Offline alpard

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Cross Modulation Problem with Active Antenna?
« on: July 02, 2020, 1540 UTC »
Hi All

I got this new active antenna, and it has a little element of about a foot, and then it connects to a RF amp, and then to the radio.
The RF amp is powered by 12V DC.

The problem is that when the radio is tuned to some frequencies such as 3310 kHz, it suffers from bad intermodulation / cross modulation. It hears some very strong signals what sounds like from MW band, and there are about 3 or 4 signals coming out together.

When the RF amp of the active antenna is switched off, the inter modulation clears.

Is this active antenna faulty? Or something else might be issue here?

Thanks 73s
Al.

Offline ChrisSmolinski

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Re: Cross Modulation Problem with Active Antenna?
« Reply #1 on: July 02, 2020, 1635 UTC »
Which active antenna?  Which amp? Give us some info.

Any amplifier will eventually cause images/etc when overloaded. By putting an RF amp after an antenna that already has an amp, you increase the odds of that happening.

Also, an amplifier amplifies everything, signal and noise. And then adds more noise. Many/most radios have sufficient sensitivity anyway. So you are not gaining anything by lots of amplification. It may make the S meter read higher, but that's it. You may well be making this worse, even before you start to get overloading and images.
Chris Smolinski
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Offline alpard

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Re: Cross Modulation Problem with Active Antenna?
« Reply #2 on: July 02, 2020, 1714 UTC »
My ICOM IC-751A was getting intermodulation from this antenna on some frequencies. The one I remember is 3310kHz = hears 3-4 strong MW stations.

https://www.ebay.co.uk/itm/Compact-Coil-Active-Antenna-DC-500kHz-30MHz-Made-in-Dorset-UK/283347768970?hash=item41f8d7d68a:g:fisAAOSwuOdcRf-I

When I take out the white pipe element, and connect the outside wire into the RF amp, it doesn't have the intermodulation problems. The antenna seems well made, and gets very good reviews.


« Last Edit: July 02, 2020, 1715 UTC by alpard »

Offline ChrisSmolinski

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Re: Cross Modulation Problem with Active Antenna?
« Reply #3 on: July 02, 2020, 1811 UTC »
Any small active antenna is inherently a compromise. It probably has a lot of gain. Why are you connecting it to another amp before your radio?
Chris Smolinski
Westminster, MD
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Offline Stretchyman

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Re: Cross Modulation Problem with Active Antenna?
« Reply #4 on: July 02, 2020, 1850 UTC »
There appears to be some confusion, probably me...

The RF amp is inide the white cylinder is it not?

The box is nothing more than a bias T containing a single cap and possibly a choke so not sure what you mean about turning off the amp, you can't unless as Chris has suggested you're using another amp as well.

Please clarify..!

Regards

Stretchy.
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Offline alpard

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Re: Cross Modulation Problem with Active Antenna?
« Reply #5 on: July 02, 2020, 1852 UTC »
I am connecting the same RF Amp, but different antenna element = external long wire, and the intermodulation problems clear.

The intermodulation problem seems to be caused by the white pipe element = coiled antenna element  of the active antenna.

Offline alpard

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Re: Cross Modulation Problem with Active Antenna?
« Reply #6 on: July 02, 2020, 1854 UTC »
Aha - maybe the RF amp is in the white pipe element?
I didn't know that.

I thought the RF amp was the little black box with the power input and output to the radio.
I am still not sure.

But when I connect the external long wire into the black box with the power input, and power on in line to the IC-751A, it makes huge difference in sensitivity. It gives about 2+ S points. Because of that I thought the little black box was the RF amp.
« Last Edit: July 02, 2020, 1856 UTC by alpard »

Offline ChrisSmolinski

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Re: Cross Modulation Problem with Active Antenna?
« Reply #7 on: July 02, 2020, 1857 UTC »
Aha - maybe the RF amp is in the white pipe element?
I didn't know that.

I thought the RF amp was the little black box with the power input and output to the radio.


That explains it.

Quote
When I take out the white pipe element, and connect the outside wire into the RF amp, it doesn't have the intermodulation problems.

So two things here....

1. You're connecting an outside (longwire?) antenna to the DC inserter. Which effectively does nothing other than just connect your outside antenna to your radio.

2. If you have an outside antenna, why use a small active antenna? (2a. Where is the active antenna located, inside the house or outside?)
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
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Offline alpard

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Re: Cross Modulation Problem with Active Antenna?
« Reply #8 on: July 02, 2020, 1901 UTC »
Aha - maybe the RF amp is in the white pipe element?
I didn't know that.

I thought the RF amp was the little black box with the power input and output to the radio.


That explains it.

Quote
When I take out the white pipe element, and connect the outside wire into the RF amp, it doesn't have the intermodulation problems.

So two things here....

1. You're connecting an outside (longwire?) antenna to the DC inserter. Which effectively does nothing other than just connect your outside antenna to your radio.

2. If you have an outside antenna, why use a small active antenna? (2a. Where is the active antenna located, inside the house or outside?)

The active antenna was for the other room in downstairs. But I was testing it out on upstairs side by side with the external wire antenna.

Offline alpard

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Re: Cross Modulation Problem with Active Antenna?
« Reply #9 on: July 02, 2020, 1902 UTC »
But the white pipe element is connected by a normal 50 ohms coax to the DC box (RF AMP, I thought), I thought there is no way that it could be powered?   Therefore it couldn't house the RF Amp in there.

Offline alpard

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Re: Cross Modulation Problem with Active Antenna?
« Reply #10 on: July 02, 2020, 1933 UTC »
Yeah maybe you are right, the amp is in the element. When the element is connected to the bias-T, and the IC-751A reception is 59+ on most signals. I had to switch on the attenuator on the IC-751A.

This Active Antenna RF amp is super sensitive.  But I am not sure if it is a good thing with some intermodulation problems on the reception.
Thanks for your pointing out on the problems here with the RF Amp mistaken identity. I have never used Active Antenna ever before. This is the first time, and it was for the downstairs another room that I am going to use as a radio room. :D

Offline alpard

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Re: Cross Modulation Problem with Active Antenna?
« Reply #11 on: July 03, 2020, 0719 UTC »
This active antenna is very sensitive. Compared to the outside LW, it is about 4 S-points stronger on all the signals. But as you say, it also amplifies the noise too. So it might be not ideal for copying weak DX signals.

Offline ChrisSmolinski

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Re: Cross Modulation Problem with Active Antenna?
« Reply #12 on: July 03, 2020, 1308 UTC »
This active antenna is very sensitive. Compared to the outside LW, it is about 4 S-points stronger on all the signals. But as you say, it also amplifies the noise too. So it might be not ideal for copying weak DX signals.

What you can do is listen to how a station, especially the weaker DX targets you are interested in, comes in on each antenna. Don't even look at the S meter. Just listen to the signal and decide which is better. You may be surprised.
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
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Offline alpard

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Re: Cross Modulation Problem with Active Antenna?
« Reply #13 on: July 04, 2020, 0901 UTC »
This active antenna is very sensitive. Compared to the outside LW, it is about 4 S-points stronger on all the signals. But as you say, it also amplifies the noise too. So it might be not ideal for copying weak DX signals.

What you can do is listen to how a station, especially the weaker DX targets you are interested in, comes in on each antenna. Don't even look at the S meter. Just listen to the signal and decide which is better. You may be surprised.

Yeah, that sounds the way to go.  Right now, the active antenna is very noisy on reception compared to outside LW. Not good for the weak DX signals at all.

I read about the active antenna again, and it says that the antenna must be powered by some sort of linear Power Supply. But all my power supply is switching mode psu, and that might cause excess noise and intermodulation?  I will try to get a 12V rechargeable battery from the shed, and will try with that to see if the situation will improve.
« Last Edit: July 04, 2020, 1631 UTC by alpard »

Offline Brian

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Re: Cross Modulation Problem with Active Antenna?
« Reply #14 on: July 05, 2020, 1233 UTC »

I read about the active antenna again, and it says that the antenna must be powered by some sort of linear Power Supply. But all my power supply is switching mode psu, and that might cause excess noise and intermodulation?
  I will try to get a 12V rechargeable battery from the shed, and will try with that to see if the situation will improve.

Shouldn't have to say this but get rid of the SMPSU asap!