Building an external ferrite rod aerial - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

paulsherwin · 17th Aug 2004, 12:41 pm

Just for everyone's info, I found the original version of this so useful that I built another, this time using a tiny ferrite rod from a scrap ghettoblaster and a MW only coil. Sensitivity is very good, although not as good as the first one which used a huge rod. I tried a range of silicon transistors from hfe 120 to hfe 400 and all worked fine without stability problems. 3V supply.

Shunting the coil with a 1nF cap gives quite good performance on LW, with R4 and RTE1 coming in well, though I need to experiment with the cap value a bit more.

This fits in a tiny wooden box about 3 " x 2 " x 2 " .

Best regards, Paul

30sradio · 5th Nov 2006, 3:07 pm

Sorry to drag up this old thread; I found it on considering the very same!
Has Mr. Sherwin or anyone else had much/any success with this?

paulsherwin · 5th Nov 2006, 4:22 pm

Quote:

Originally Posted by 30sradio

Sorry to drag up this old thread; I found it on considering the very same!
Has Mr. Sherwin or anyone else had much/any success with this?

Wow, this thread has risen from the dead! I'd assumed it had been purged.

Go ahead and build one, it's very easy. There can be stability problems with some radios, presumably because of the aerial coupling arrangements, but in most cases performance is very good, as it should be considering a stage of tuned RF amplification has been added. I found a 3V supply worked best.

Paul

30sradio · 5th Nov 2006, 11:01 pm

Many thanks for your encouragement, Paul. I'll try and knock one up this week.

30sradio · 8th Nov 2006, 11:38 pm

Well, I constructed one today.
I used a BC108 plus the other parts as per Paul's cct. diagram. Initially I just used lengths of single to connect via wander plugs to the radio. It oscillated on MW but was fine on LW.
I then changed to a length of coax.
The results, connected to my 1938 Cossor (which OOI has an RF amp) R4 LW is superb

MW is a bit ropey; (faint) I was hoping for better results on MW; but I'm delighted with R4LW.

Any comments or observations greatly appreciated.

paulsherwin · 9th Nov 2006, 12:17 pm

Quote:

Originally Posted by 30sradio

MW is a bit ropey; (faint) I was hoping for better results on MW; but I'm delighted with R4LW.

It sounds as if something is wrong here. Just to clarify, you should be using a LW coil for LW and a MW coil for MW

There should be a very obvious peak to the signal as you tune the capacitor. The ones I have built work very well on MW, pulling in very faint stations indeed (I remember listening to BBC Radio Cambridge on MW once, normally completely inaudible here).

Good luck, Paul

30sradio · 9th Nov 2006, 4:37 pm

Thanks for your reply, Paul.

I knocked this up last night before bed.

In the cold light of day, I've realised I missed out a wavechange switch

Off now to rectify

Thanks again, Rob.

paulsherwin · 9th Nov 2006, 5:24 pm

I'm sure that will sort it out.

Although the original circuit works well, it should really have a 1nF capacitor across the electrolytic to decouple the RF properly. In fact you probably don't need the electrolytic at all but it does no harm.

It's important to minimise the length of any unscreened wiring connected to the transistor collector as this radiates RF which is picked up by the rod and causes instability. If I were building another one I'd use short transistor leads, short leads on the coupling capacitor, and connect the capacitor to any socket with coax.

None of the components are in any way critical.

Paul

30sradio · 9th Nov 2006, 7:29 pm

I've now sorted the W/C switch (well, a test-lead with crocs each end for now!) and results are truly excellent.
I've tried it on a couple of other radios and am very pleased.

A great project, Paul.

17th Aug 2004, 12:41 pm	#21
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,957	Re: Building an external ferrite rod aerial Just for everyone's info, I found the original version of this so useful that I built another, this time using a tiny ferrite rod from a scrap ghettoblaster and a MW only coil. Sensitivity is very good, although not as good as the first one which used a huge rod. I tried a range of silicon transistors from hfe 120 to hfe 400 and all worked fine without stability problems. 3V supply. Shunting the coil with a 1nF cap gives quite good performance on LW, with R4 and RTE1 coming in well, though I need to experiment with the cap value a bit more. This fits in a tiny wooden box about 3 " x 2 " x 2 " . Best regards, Paul

5th Nov 2006, 3:07 pm	#22
30sradio Retired Dormant Member Join Date: Oct 2006 Location: Cambridge or Kent Posts: 8	Re: Building an external ferrite rod aerial Sorry to drag up this old thread; I found it on considering the very same! Has Mr. Sherwin or anyone else had much/any success with this? Last edited by 30sradio; 5th Nov 2006 at 3:08 pm. Reason: typo

5th Nov 2006, 11:01 pm	#24
30sradio Retired Dormant Member Join Date: Oct 2006 Location: Cambridge or Kent Posts: 8	Re: Building an external ferrite rod aerial Many thanks for your encouragement, Paul. I'll try and knock one up this week.

8th Nov 2006, 11:38 pm	#25
30sradio Retired Dormant Member Join Date: Oct 2006 Location: Cambridge or Kent Posts: 8	Re: Building an external ferrite rod aerial Well, I constructed one today. I used a BC108 plus the other parts as per Paul's cct. diagram. Initially I just used lengths of single to connect via wander plugs to the radio. It oscillated on MW but was fine on LW. I then changed to a length of coax. The results, connected to my 1938 Cossor (which OOI has an RF amp) R4 LW is superb MW is a bit ropey; (faint) I was hoping for better results on MW; but I'm delighted with R4LW. Any comments or observations greatly appreciated.

9th Nov 2006, 4:37 pm	#27
30sradio Retired Dormant Member Join Date: Oct 2006 Location: Cambridge or Kent Posts: 8	Re: Building an external ferrite rod aerial Thanks for your reply, Paul. I knocked this up last night before bed. In the cold light of day, I've realised I missed out a wavechange switch Off now to rectify Thanks again, Rob.

9th Nov 2006, 5:24 pm	#28
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,957	Re: Building an external ferrite rod aerial I'm sure that will sort it out. Although the original circuit works well, it should really have a 1nF capacitor across the electrolytic to decouple the RF properly. In fact you probably don't need the electrolytic at all but it does no harm. It's important to minimise the length of any unscreened wiring connected to the transistor collector as this radiates RF which is picked up by the rod and causes instability. If I were building another one I'd use short transistor leads, short leads on the coupling capacitor, and connect the capacitor to any socket with coax. None of the components are in any way critical. Paul

9th Nov 2006, 7:29 pm	#29
30sradio Retired Dormant Member Join Date: Oct 2006 Location: Cambridge or Kent Posts: 8	Re: Building an external ferrite rod aerial I've now sorted the W/C switch (well, a test-lead with crocs each end for now!) and results are truly excellent. I've tried it on a couple of other radios and am very pleased. A great project, Paul.