External aerials for FM/AM

[jhockridge]

Good afternoon,

Does anyone know the best way to set up a FM and AM aerial on the roof of a normal house? I know you can buy DAB aerials easily, but I am not after that. They would be feeding twin outlets, one in the office and one where the main HIFi is. I would be really interested in having a go at constructing my own aerial.

Thanks, James.

[G6Tanuki]

What do you want to receive, FM-wise? If you're after general listening to a wide range of local(ish) stations then a simple vertical dipole is the easy way to do it. If your interests are hi-fi from something like radio3 then a directional antenna will reduce the effects of multipath-distortion and 'burblies'. If you're interested in FM-DX then add a rotator.

For AM, what receiver will you be using? Does it have separate AM antenna- and earth/ground terminals? if so, you could _try_ using a longwire - 30 to 50 feet of wire strung to a remote post/tree - but you run the risk of it picking up lots of noise. Again, what do you want to receive on AM? Local stations or more-remote stuff. You're pretty close to the transmitter used for Radio Caroline!

[Nuvistor]

For FM this is a good web site, plenty of information.
https://www.aerialsandtv.com/fmanddabradio.html

For AM.
https://www.vintage-radio.net/forum/...d.php?t=157926

Whatever you do, you need to know the answers to G6tanuki’s questions

[jhockridge]

Thank you chaps. I am interested in how far I can receive AM and LW signals from. For FM it is more about listening. A directional yagi? type antenna pointed at the nearest mast is what you are suggesting?

[Nuvistor]

A simple wire aerial for AM is easy to try but if you do get lots of inference from the electrical items every home has then a magnetic loop may be worth a try.

A directional yagi at the nearest FM transmitter should do the job, unless you are in a difficult reception area it may only require a couple of elements.

[Mike. Watterson]

MW is local during day and can be as far as North America at night.
LW isn't so much affected by day & night as it's ground wave.
You need a loop for MW & LW otherwise interference is the limiting factor.
Do note that LW, MW and SW is more correct as 110MHz to 132MHz approx is AM.
CB (26 to 27MHz and varies by country) can be AM, FM or SSB. Technically SW is at the edge of SW.
SW can be AM, pilot tone SSB, SSB or DRM (if it's still running).
RTE has done DRM transmissions on LW, the normally AM 252KHz

A 75cm whip for local* FM and a Yagi (vertical) for DX (distant) Broadcast FM. Amateur VHF on 144MHz to 146MHz tends to use a whip for local FM and a horizontal yagi for DX of SSB or various digital signals. I have reached Belfast, Wales and Devon from here 9km west of Limerick City on 144MHz band VHF. I've reached Norway & Switzerland on 5W 29MHz FM when conditions are right. Also Orkney Is with a 6ft whip in the living room ONCE on SSB, about 12W using a military set on 1.8MHz.

So Distance (DX) depends on band, time of day, weather conditions, local interference and aerials.

(* I actually receive "local" stations on Broadcast FM using a roof Discone from Cork City and transmitters in the South East and I'm in the Mid West. I receive Limerick 95 from the Dublin side of Portliose on the car radio. Because VHF-FM Broadcast stations are designed to work on a portable indoors.)

[Station X]

Originally Posted by jhockridge

For FM it is more about listening. A directional yagi? type antenna pointed at the nearest mast is what you are suggesting?

Given that the nearest mast is in Manningtree I wouldn't think a multi-element aerial would be needed.

[G6Tanuki]

Sometimes the "simplest" situations are the most-complex. Without knowing what local random-signal-reflectors there are in the area -- which can deliver loads of multipath-distortion -- you just can't tell.

A decent directional antenna isn't always about gain, it can often be about excluding nuisance-reflections from the local environment.

Sidenote: given that the original poster has expressed an interest in listening to 'distant' AM/MW broadcast-stations, if he's in an 'urban' environment with near-neighbours whose houses will likely be filled with noise-generators, a classic 'longwire' antenna as I suggested may not be the best solution - some sort of tuned/balanced loop could well be better.

But that assumes his AM/MW radio has suitable 'external antenna' connection terminals.

[Mike. Watterson]

Setting a radio with a ferrite rod inside a large tuned loop works very well.

[SteveCG]

jhockridge,

Do you already have receivers? If so what makes and models are they please. Knowing this it could be helpful to us to advise on aerial types (eg a lot of post 2000 tuner/CD/ Amplifiers use FM/AM modules - on these the AM inputs are designed to have a loop aerial connected).

[nicam49]

Surely for fm reception, a simple omni-directional aerial will fit the bill for sure. Cheap as chips from toolstation and screwfix.

[stevehertz]

Just read all the stuff about aerials on the web site link already given to you: https://www.aerialsandtv.com/fmanddabradio.html

It's a wealth of information and it is very practical based/biased. Not so much theoretical BS and more about what actually works in the real world.

[Boulevardier]

Thanks Steve - very useful link, full of down-to-earth practical tips. Sort of things none of the theory will tell you...

Mike

[jsbhavsar]

If the receiver's FM aerial socket is just a single terminal, a 57 inches long insulated wire connected and left hanging there can still receive good FM signal.

For MW the length of the wire should be much beyond 20 to 25 metres and the far end of the wire must go outside the house structure to receive well.

[G8HQP Dave]

Originally Posted by stevehertz

It's a wealth of information and it is very practical based/biased. Not so much theoretical BS and more about what actually works in the real world.

Yes, generally helpful. It gets the purpose of baluns wrong when it says they are for impedance matching, but even some radio amateurs get that wrong. A wrong theoretical understanding then leads to poor practical advice when they say that a dipole does not need a balun because it is already 75 ohms.

The truth is that a balun is meant to couple the balanced antenna to an unbalanced feeder, and thus reduce or eliminate common-mode current - which on reception causes more noise/interference pickup. A dipole needs a balun just as much as a Yagi, however in both cases if you have strong signals and weak interference you can get away without one.

[Nuvistor]

While a balun is for connecting a balanced to unbalanced line, there are 1:1 Baluns and 4 :1 baluns, possibly others.

The 4:1 would provide both unbalanced to balance and also impedance matching.
A 4:1 very often used with a folded dipole to coax.

[Synchrodyne]

Originally Posted by G8HQP Dave

Yes, generally helpful. It gets the purpose of baluns wrong when it says they are for impedance matching, but even some radio amateurs get that wrong. A wrong theoretical understanding then leads to poor practical advice when they say that a dipole does not need a balun because it is already 75 ohms.

The truth is that a balun is meant to couple the balanced antenna to an unbalanced feeder, and thus reduce or eliminate common-mode current - which on reception causes more noise/interference pickup. A dipole needs a balun just as much as a Yagi, however in both cases if you have strong signals and weak interference you can get away without one.

I had noticed that error as well. What struck me was not so much the not-uncommon error about the fundamental nature of a balun, but the adamant nature of the statement.

“Being a half wave dipole it naturally has a 75Ω impedance, i.e. matched to the cable and (most) receivers for maximum signal transfer. Thus an open dipole should not have a balun, if it does there`s something wrong somewhere !”

The level of confusion is apparent from this statement:

“Decent Yagi aerials have a "balun" in the dipole which is usually a small PCB built into the dipole assembly and it`s designed to maximise the signal by impedance matching the dipole to the cable, hence the name from the term balanced unbalanced. Baluns are reputed to help eliminate impulse interference which can be one of the causes of intermittent blocking or pixellation on digital signals.”

The first sentence is really a non-sequitur, conflating impedance transformation and balance state transformation. The second sentence, taken with the previous adamant statement, suggests that the writer, whilst aware of this particular benefit that stems from using a balun, nevertheless does not know why it is so.

Originally Posted by Nuvistor

While a balun is for connecting a balanced to unbalanced line, there are 1:1 Baluns and 4 :1 baluns, possibly others.

The 4:1 would provide both unbalanced to balance and also impedance matching.
A 4:1 very often used with a folded dipole to coax.

I suspect that the 4:1, 300R-balanced-to/from-75R-unbalanced is the most common device that goes by the “balun” name, at least in domestic receiving applications. It is simultaneously a balun and an impedance transformer. But the latter function seems to have loomed much larger, and that perhaps explains why the term balun has become synonymous with impedance transformation, to the extent that some observers infer that if no impedance transformation is required, then concomitantly no balun is required.

All of that said, a dipole connected directly to a coaxial cable just looks plain wrong – the two halves are not being treated equally!


Cheers,

[Herald1360]

The site claims to welcome feedback- it even claims "scholars are welcome". Might be worth putting that to the test......

[lesmw0sec]

Regarding FM - it all depends upon the local signal strength. In the Swanage area where the signal was excellent, a bit of wire stuck into the aerial socket was fine. For better reception and where the set had an unbalanced input, we used to use twin flex to the attic or along a picture rail, with the ends divided to form a half-wave dipole (total length (feet) being 468/freq. in Mc/s). Easy stuff to try and see.

[colourking]

Originally Posted by Mike. Watterson

LW isn't so much affected by day & night as it's ground wave.

Try telling that to listeners of RTE's LW on 252kHz (or 1190m on vintage radios), in a good portion of Ireland as well as Britain its destroyed by co-channel interference from Algeria at night. The obvious solution being that RTE seek to move it up a 'channel' to 261kHz for the remainder of its life, though looks like they are not interested in pursuing this