QRM antenna

This is the place to discuss any general CB radio related topics. Getting started, installations, operation etc.
Post Reply
User avatar
johnkyrle
Regular
Regular
Posts: 87
Joined: 25 Apr 2016, 20:24
Call Sign: 26TM3375
Location: South Gloucestershire
Contact:

QRM antenna

Post by johnkyrle » 10 Mar 2017, 19:58

I have read somewhere but for the life of me cannot find it again that you can construct a loop antenna that attracts QRM signals.I have just installed an IMAX 2000 and have noticed that the QRM is higher (obviously) compared to my Sigma Venom 5/8,due to the IMAX being more efficient.
Anyone any ideas?Apart from the QRM eliminator from Germany which,at the moment,is out of my price range.

User avatar
Admiral
Legend
Legend
Posts: 9707
Joined: 08 Mar 2011, 21:20
Call Sign: 26TM157
Location: MK-UK

Re: QRM antenna

Post by Admiral » 10 Mar 2017, 20:33

Good luck getting an answer to your question as it makes no sense to me whatsoever.
Winner of the 2017 IBTL 'Summer Sizzler' competition

User avatar
WeatherWatcher
Radio Addict
Radio Addict
Posts: 795
Joined: 16 Oct 2016, 11:40
Location: where ever APRS says I am

Re: QRM antenna

Post by WeatherWatcher » 10 Mar 2017, 20:37

viewtopic.php?f=2&t=52872
This thread might help

paulears
Radio Addict
Radio Addict
Posts: 624
Joined: 10 Jun 2007, 22:41
Call Sign: G4RMT
Location: North East Suffolk
Contact:

Re: QRM antenna

Post by paulears » 10 Mar 2017, 21:25

Remember that aerials don't attract anything - so perhaps 'capture' would be a better word. QRM is just unwanted noise, and the opposite is the wanted signals. Any antenna design should work as well as possible at capturing whatever is in the ether. They have performance that can be measured - gain, bandwidth and some have directional properties. Loops are a good example - they're physically small normally, so not that good an aerial one would assume, but they have very deep nulls - ships used them for years for direction finding. Twist them around till the signal disappears and then you'd know exactly where the transmission came from. An omni design might get swamped by all sorts of noise you don't want - which is where it might appear it attracts interference - it doesn't it captures it.

That QRM eliminator is a simple gadget that nulls out unwanted noise by phase cancellation as said already. If it works, that's great - but it isn't that clever.

If the Imax picks up more QRM, it will also pick up more signal. This is simply signal to noise ratio. If two aerials produce the same wanted signal but different unwanted noise, then they must be different in polar pattern, or have different reception angles - ones that capture from the noise direction and not so much from the wanted signal direction. Noise from the local welding workshop might be horizontal, but the weak signal from abroad coming down at an angle?

DaveP
Regular
Regular
Posts: 22
Joined: 14 Oct 2016, 18:40
Call Sign: 2E0DDF

Re: QRM antenna

Post by DaveP » 17 Mar 2017, 20:00

Don't waste your cash or time getting a QRM eliminator, all it does is act like a pre-amp but in reverse, so not only will it cut down QRM but also readable signal. You have not said what band you are on or what coax you are using?? Have you fitted a balun?? If not very easy to fit and make your own, once done it should cut down the unwanted noise dramatically. I am assuming you are on 11 meters, un-screw the coax from the antenna and using a coke bottle, tin of beans or any circular object with the diameter of 4 to 4.5 inches wind the coax round tightly twice as close to the plug as possible, cairfully remove the former and tape you balun together, so that the end with the plug starts and the remaining coax is directly below. Refit the plug to the antenna and check, if you still have high QRM do one extra turn, no more than 3 or you start to make a choke which will reduce your signal by .5 DB by every turn. If that doesn't work then invest in better shielded coax, such as aircell 7 or for less money but in my opinion a better coax ultra flex 7.

Good luck. 73s Dave
Yeasu FT-897 wide banded, MD100 mic, Yeasu FT-857, Yeasu FT-8800r, Diamond X300 2/70, A pair of phased shunt type loops for 2/70. 6 meter shunt type loop, Windom 20m to 10m long wire, Antron 99s for 10 and 12 meters. As you can see Im a Yeasu Man and loopy to boot. :D

User avatar
mof000
Veteran
Veteran
Posts: 4190
Joined: 20 Jan 2008, 13:28
Location: United States of the UK

Re: QRM antenna

Post by mof000 » 17 Mar 2017, 21:02

DaveP wrote:Don't waste your cash or time getting a QRM eliminator, all it does is act like a pre-amp but in reverse, so not only will it cut down QRM but also readable signal. You have not said what band you are on or what coax you are using?? Have you fitted a balun?? If not very easy to fit and make your own, once done it should cut down the unwanted noise dramatically. I am assuming you are on 11 meters, un-screw the coax from the antenna and using a coke bottle, tin of beans or any circular object with the diameter of 4 to 4.5 inches wind the coax round tightly twice as close to the plug as possible, cairfully remove the former and tape you balun together, so that the end with the plug starts and the remaining coax is directly below. Refit the plug to the antenna and check, if you still have high QRM do one extra turn, no more than 3 or you start to make a choke which will reduce your signal by .5 DB by every turn. If that doesn't work then invest in better shielded coax, such as aircell 7 or for less money but in my opinion a better coax ultra flex 7.

Good luck. 73s Dave
My brother bought the Timewave ANC 4 to help with PLT. It certainly got rid of the noise AND every other wanted signal!
I have read reports that they (ANC) are good for plasma interference.
I'll pay whatever i think it's worth...:-)

wa10
Top Poster
Top Poster
Posts: 1136
Joined: 19 Feb 2008, 00:37

Re: QRM antenna

Post by wa10 » 17 Mar 2017, 23:19

A look at any reputable source of info pertaining to winding chokes or using ferite on coax cable will confirm that chokes DO NOT
cause loss,
lower your power
filter harmonics
attenuate your signal by .5db per turn,
act like a low pass filter

chokes don't do any of that, they don't effect signals inside the coax at all,

where do people get their made up science from.

Post Reply

Who is online

Users browsing this forum: No registered users and 6 guests