MARPA AM Reception Issues

Getting a big Hum on the AM reception.

The unit is in the basement. the Antenna is on the roof fed direct to basement and then going into splitter in basement which then feeds all the TV points.

the instructions mention putting the cut in the shield well away from the unit.

this is a but hard as the unit and the spitter are right next to each other.

I connected the splitter to the MARPA via 10 meters of RG6 Quad Shield and cut the shied at 5meters and ran the cable 5 meters away and back to the cabinet.

Still getting HUM

AM reception in the basement is poor.

There are 2 unused RG6 runs to the roof space I was thinking of running the AM up and back and put the shield cut in the roof space. Would this help ??

Is there a dedicated AM/FM antenna that I could just buy and put in the roof and not connect to the TV system at all. Do you still need to cut the shield ?

Charles

 

Hi Charles

The reason for cutting the shield to so there is not a closed (well 75 ohm) loop. You will get no reception if there is not a break in the 'loop' somewhere. AM can be tricky at the best of times.

 

AM antennas

I am having alot of trouble finding and AM/FM antenna

There are plenty of AM Only

http://www.radioparts.com.au/ProdVi...sc=CLOOPHDMATCH+COMMERCIAL+GRADE+LOOP+ANTENNA

And Plenty of FM Only

http://www.radioparts.com.au/ProdVi...42056&ProdDesc=Y3FM+FM+RADIO+ANTENNA+/+AERIAL


But not a combined one

just these car ones

http://www.radioparts.com.au/ProdVi...9093&ProdDesc=M2P+1520MM+AM+/+FM+RADIO+AERIAL

but they are not using RG6


Antenna experts I have been talking to say there should be seperate FM and AM antenna sockets on the equipment. But I have been telling them there is not.


Could I use both of these

http://www.radioparts.com.au/ProdVi...sc=CLOOPHDMATCH+COMMERCIAL+GRADE+LOOP+ANTENNA

http://www.radioparts.com.au/ProdVi...42056&ProdDesc=Y3FM+FM+RADIO+ANTENNA+/+AERIAL

and join together with a combiner and connect to the antenna socket.


There is also this

http://www.amradioantennas.com/Leaflets/PK's HD Wideband AM Loop C-LOOP-HDWAM-FM.pdf

with FM Combiner


Would I still need to cut the RG6 Cable ??

Charles

 

FM radio broadcasts are in the VHF band, with wavelengths of approximately 3 metres, while AM broadcasts are in the LF band, with wavelengths closer to 300 metres.

For a simple electrical antenna to be effective in receiving a signal, it needs to be a considerable fraction of a wavelength, usually at least 1/10 of a wavelength. That's still 30 metres for AM!

If you connect a long antenna for AM to the input to a receiver, you can still connect a short FM antenna as well, in parallel, and depending on exact wire lengths, these should not interfere with each other much. If you want to, you can make a simple 'diplexer' to isolate the antennas by simply connecting the FM antenna in series with a small capacitor (47pF), and the AM antenna in series with a small inductor (4.7 microHehries) at the connector so that the signals are not affected by the other antenna (a similar network actually exists inside the MARPA unit to separate the signals again for processing). This is common practice in shortwave receivers connected to arrays of antennas (diplexers become more complex, but the idea is the same).

Ok, so how do they make tiny AM radios with no external antenna, that can pick up stations (at night) half way across the planet? The answer is to use a magnetic antenna. Using a magnetic antenna reduces pick-up of electrical noise from local sources, so can result in very good AM reception. The AM 'loop' antenna you provided links to is probably a magnetic antenna with an air core. You can make a more compact one yourself if you can find a ferrite bar (approximately 100mm long), and wrap 20 to 50 turns of wire around it (depending on the bar), then connect one end of the wire to the antenna input, and the other end of the wire to the screen, you should be able to get very good AM reception. An FM antenna can be connected in parallel without any significant affect. Note that magnetic antennas tend to be quite directional, so there will be an optimal position for your favorite stations.

Loop antennas are not used in cars because:
a) cars are usually metal, which screens the magnetic and electric fields, so an external antenna is better
b) simple magnetic antennas are directional, so signals would fade every time you turned a corner.

Just a few ideas there.

 

No AM reception with a Loop Antenna

Have one of these installed.

http://www.radioparts.com.au/ProdVi...sc=CLOOPHDMATCH+COMMERCIAL+GRADE+LOOP+ANTENNA


Tuned for 20 Meters of RG6 Quad.


Have 20 meters of RG6 Quad and connected to MARPA.

Get nothing but static and white noise regardless of rotation.


Cut the shield about 2 meters from the MARPA get AP reception but lots of HUMM no better that it was when I was using the TV antenna

Cut the shield at the antenna and get NO reception.

The antenna manufacture wants me to bring the MARPA to his office to test.

(as you know not that easy need amps cbus etc)

On the screen there is a bargraph under the tuner frequency. Does this represent the signal strength ??

any chance of getting the circuit of what is between the Fconnector on the back and the internal tuners.

Any ideas why this does not work.


Charles

 

Hello Charles
Are you able to grab another AV reciever or even just an AM radio and try it to rule out a local problem instead of taking all the equipment in.
Also have you considered using a DAB digital radio as a source unit.
If you have DAB in the area and the stations you want are available, the sound quality you will get will be so far superior to the best you could achieve with AM that I don't think you would even consider trying to get it to work.
I did set up a small DAB clock radio for my dad in his home about 100km from Brisbane.
http://www.pure.com/au/products/product.asp?Product=VL-60907&Category=Bedroom
He went and bought it without much thought and when he got it home it had zero reception.
I was able to connect the short lead antenna that it came with to the pin of a wall F socket.
The reception then became rock solid and he very much enjoyed the available content.
I suppose thats not so relevant since nowdays you can pick up a DAB radio with a PAL connector.
Even a small relatively cheap unit like that plugged into some spare inputs will sound far superior to AM.
But that doesnt help you much with any sort of automation I suppose.
I have never done this but perhaps you could locate the radio in a convenient location and run the output to the amp with RG6 or try and find a DAB radio which powers up to it's last state and leave it running in the basement.
One other thing which may be of interest was that on this job site's TV antenna there is a masthead amplifier and it has a filter which blocks FM radio frequencies.
It does seem to block regular FM tuners but the DAB seems to pass through without a problem.
Regards
Daryl

 

The location is fine. Sony Tuner with a small loop at the back works fine (no humm) in the basement. with out external antenna

 

Attached the PK Loop Antenna to the SONY tuner and get perfect reception at the end of the RG6Quad.

 

Does any one else have a Matrix Switcher 2.0 with AM working Clearly.

Spent the morning with Paul and PK AM loop Antennas and he could not get decent AM reception from my Matrix Switcher 2.0 with all his knowledge and test equipment.

Charles

 

We are planning to spend some time investigating this issue next week. We will let you know what we find.

 

AM Antenna reception

Thank you. Paul at PK loop Antennas is willing to help also. His contact details are in my case and I sent them in a PM to Conformist. Paul is very experenced in this area dont let his offer to help be missed.

 

What did you find

What did you find ?

 

We have not completed our investigations yet, but we did find that it was difficult to get good performance from the AM radio when the signal strength is weak. Using a commercially available loop antenna was not suitable. The best performance seems to be from a marine whip antenna.

We will provide some more specific advice when our investigation is complete, which should be soon.

 

Are you saying with a strong signal you got crystal clear, no humm no buzz or pumping AM reception ??


When I was at PKloop Lab even with a AM signal generator locked on 774Mhz playing a tone with variable gain directly connected you could not get a good signal, so there is something fundamentally wrong. (FM on the other hand was perfect.)

If a signal generator directly connected does not work then no antenna is going to work.

I wish one of you guys would talk to Paul at PK loop (PKloops @ bigpond net.au . He did a lot of other tests on this radio and had good understanding of what is wrong and suggestions for you that went way over my head.

He feels there is something fundamentally wrong with the Splitter between the two tuners and AM /FM duplexer.

He has vast experience in AM product development and is willing to help.

He just wanted to see the circuit diagram from the Antenna plug on the back to the feed on the tuner chips. Is there any chance you could send him that. I am sure you will get useful information. He knows a lot about AM tuner chips and the software that runs on them.

Having to cut the Shield makes the FM go from Perfect to OK. This cutting the shield makes no sense to Paul and he showed me the effect on the test instruments this made to the FM signal go from Perfect to OK. So you cut the shield to get AM and then your FM gets affected.

If I connect a Sony tuner to a duplexer with PKloop antenna on the AM and FM antenna on the FM I get Perfect reception AM and FM.


Charles