Help:can I use wire antenna to replace magnetic rod antenna in receiving MW signal?

[kenshinjia]

Hi everyone!

I am building a simiple DRM receiver's front end.I am using a simple long wire as the antenna to lead the signal to the mixer SA612A.By using the DreaM software,I can only hear two AM programs in MW (there is no DRM in my city,I just use it to test the AM ).But using a cheap AM receiver from the market,I can hear at least 5 AM stations in MW.
I am not sure about the problem. I wonder whether it is the wire antenna cannot receive MW signal very well.Should I use the LC tuned magnetic rod antenna to replace the wire antenna ?
My front end's frame is as the picture below:

[dk8cb]

Originally posted by kenshinjia
I am using a simple long wire as the antenna ...

Hi,

I guess that you are not using what is known as a real longwire antenna. Speaking in antenna terms, a longwire is only - and only then - a longwire, if it is much longer than a wavelength! For mediumwave reception, this would mean using an antenna wire that is several hundred metres long!
So, most likely, what you are really using is a short wire antenna!
It may however still be a longwire for some shortwave frequencies, if it is sufficiently long, which I also doubt.

So how long is it? If it is just a few metres long, then make it longer or build a ferrite antenna, perhaps with a high impedance FET amplifier connected to it.

Roland

[Rene Schmitz]

Hi Kenshinjia and Roland,


A longer antenna might pick up more signal, but will also pick up more unwanted signals.

The real issue with the antenna length is that of loading. Generally a shorter antenna has a higher impedance, for a given frequency, and so the question is wether it has a lower impedance than your front end. If not, you'll load the antenna too much and the received voltage is lost in a mismatch.

Cheers,
René

[dk8cb]

Hi Rene,

you are correct, of course.

The main reason for my post was the wrong usage of the term longwire, across which I do come very often.
I think that most of the people who state that they are using a longwire antenna, do in fact just use an electically short piece of wire.

I did not want to give the impression that a longwire is necessary for mediumwave reception (although it would sometimes be nice to have one :) ).

@ Kenshinjia:

Put a single transistor high input impedance amplifier in front of your receiver or connect an impedance transforming tapped parallel tuned circuit in front of your receiver and connect your antenna wire to the "hot end" of the parallel circuit.
But before doing so, give it a try without the transistor but with just the parallel tuned circuit first.

Roland

[simone]

Hi Kenshinjia,
for MW I would prefer the ferrite rod antenna.
73, Simone

[kenshinjia]

Hi everyone!
Thanks for replying!
Yes,my wire antenna is just a few meters long which is not enough long according to Roland's suggestion.Yesterday I got a ferrite rod antenna and a variable capacitor from an old radio .I just connected it to the input of the circuit.It really worked.I could hear more clearly and get more radio programs.
So I think that my previous wire antenna is not quite good for receive the signal.But I also want to try to have a more longer one to try again.Because there is no place for the ferrite antenna on the PCB board in my previous design,it is inconvenient to add ferrite.
I will buy some more longer wire this weekend and have a try,what do you think the length I should have at least?

[kenshinjia]

Originally posted by dk8cb



So how long is it? If it is just a few metres long, then make it longer or build a ferrite antenna, perhaps with a high impedance FET amplifier connected to it.


Hi roland:
Thanks for your suggestion.My previous antenna is just a few meters long.I want to try a longer wire .What do you think I should pay attention to?

Moreover the FET amplifier you mentioned plays a role of impedence match,right?
If it is a necessary measure, I should place it between the wire antenna and the input of my circuit---my 30MHz low pass filter,right?

[Rene Schmitz]

Hi Kenshinjia and all,

The circuit you posted is ok, but I would raise R1 and R2, as shown the impedance is only 1k at radio frequencies. I would raise them both to 470k or even more.

You should consider to use an extra box for the ferrite antenna, so you can turn the antenna independantly of the receiver. And no problems with your pcb layout.

Cheers,
René

[dk8cb]

Originally posted by Mauri
I would like to build a ferrite rod loop antenna
for MW

There is a simple rule for mediumwave antennas:
The bigger, the better!

Don't expect too much from a ferrite rod antenna, better build a wire air loop that is as large as possible.
A ferrite loop, even with a FET preamplifier connected to it, will deliver much less signal than a loop of a reasonable size will do. A comparison of my 6 turns, 80 cm tuned loop to the ferrite rod antenna built into my Sangean ATS-803A receiver reveals that the loop is by far superior since it delivers a signal that is tens of dBs stronger!

Roland

[simone]

Hi Mauri,
you mentioned on the BBC 1296 kHz thread that you are using a Sony SW7600G, with this radio I get the best results (concerning audio decoding, not SNR) using the internal ferrite rod antenna even with weak signals.
Under difficult conditions results are also better than with the loop and the FRG100.
With this radio a big loop antenna is not an improvement on MW as you don´t have an external antenna jack to connect a MW antenna, so you need an antenna coupler and in the end you would use again the ferrite rod antenna.
73, Simone

[dk8cb]

Hi Simone,

a simple question: Does the external antenna input on the 7600G (I suppose, it has got one) only work on shortwave and not on mediumwave?
Or do you have to couple in in a special way (e.g. with DC path or without or with a certain DC resistance) to disable the internal ferrite antenna?

I'm asking, because I know that there are certain types of receivers where you can actually switch off the internal ferrite only by meeting certain DC conditions on the external antenna input.

Roland

[simone]

Hi Roland,
that´s exactly what I told Mauri: you have to use a coupler for MF (according to Sony). Maybe there is some other way too, but I never looked into this (as actually there was no need).
73, Simone