Internal FM aerial query

[Andrew]

Hello

I bought a Denon DRA-455 Tuner-Amp in the local BHF shop on Saturday. It's immaculate, not a scratch and works fine.

65 watts /channel and when plugged in to a Wattmeter it seems to use about 25 watts of power for normal listening so I intend to use it primarily as a TV 'soundbar'.

It came with a remote and what I presume is the original internal 'T' FM aerial that terminates in a standard FM/TV aerial plug. The back of the receiver only has a a coax socket marked 75 ohms and the pair of AM connectors. There is no additional pair of 300 ohm FM aerial inputs.

This is the query. I thought the internal FM aerials were always 300 ohm ?. There is zero resistance between the central pin of the supplied aerial plug and the outer shield. Similarly there is zero resistance between the ends of each leg of the 'T', which are 69cms long.

OTOH, the similar aerial supplied with my Denon M41DAB mini hifi has an F connector plug and here the central pin is connected to one leg of the 'T' and the shield of the connector is connected to the other leg. There is infinite resitance between the shield and the central pin of the F connector.

It seems to me that the internal aerial the came with the DRA-455 is simply wrong, and because I live

40+ miles from both of the 'nearby' main BBC transmitters (and down in a hollow) it cannot pick up any of the main BBC channels in stereo and poor quality in mono.

Luckily I have a proper 4 element yagi in the loft, tilted at 45 degrees and pointing at Rowridge and that gives perfect reception.

Still curious about the supplied T aeriel though. Shouldn't this be connected to the tuner with a balun ?

regards

Andrew

[Brian]

The T antennas supplied for VHF reception tend to be either simple / crude dipoles or folded dipoles.

Some are made from 300 ohm ribbon feeder but neither will measure 300 ohms on an ohm meter.

A simple dipole would be open circuit - it is two bits of wire not connected to each other. One to the pin and one to the shell of the connector in your case.

The folded dipole will measure near, if not, zero ohms. It is, in effect, a single conductor pin, up one leg, out, back across the top of the T, back, down, to the shell.

Living 40 miles from a transmitter, you?d be lucky to get a decent signal with either.

[Andrew]

This is the case of the aerial supplied with my Denon M41-DAB.

The one with the DRA-455 has a dead short between pin and shell.

There is zero resistance between pin and shell but the two legs of the 'T' are joined by a plastic button and from what I can gather this is just a spacer/connector. I don't think it makes contact with the copper core.

[The Natural Philosopher]

yes, the impedance refers to 'at 100MHz' and so on.

You can use a ferrite matching transformer to match a 300 ohm folded dipole to a 75 ohm input.

Or simply accept a loss of performace

indeed....

[Paul]

If you have an FM T antenna, you feed the two spade lugs to the screws on a 300 to 75 ohm Balun. You should always keep a few Baluns in the house for times like this.

My home-made TV antenna has its own Balun for this reason. As the TVs in the house are 75 ohms. There is no 300 ohm input, like in the old days.

In a Google search, look for

Balun for TV

as the TV Balun also handles FM

Here is a picture, to give a rough idea. Now, you need some wire with F-series on the end or whatever. Imagine one of these, combined with a rabbit ears with screw down lugs on its ends. On TV sets, the Balun slides right over a matching connector, for an extension-wire-free result.

[Picture] Find a bargain seller with a TV accessory wall

When a TV set comes with its own rabbit ears antenna, there is usually a Balun in the box to go with the rabbit ears.

You can take the TV set rabbit ears and TV set balun, and plug into your FM receiver 75 ohms and test. You may find sufficient FM signal, to leave the T antenna in its box. This is one of the reasons I only ever used the FM T antenna, in the car, when the car antenna snapped off from rust.

If using a TV antenna rabbit ears for FM radio, extend the telescoping section to max length.

As for the notion of "impedance", things like TV antennas are not a "constant 300 ohms" from one end of the band to the other. They can dip down to 150 ohms without too much trouble. This means the Balun is not a match <gasp>. But do not fret, because there is still plenty of signal to make things work. The Balun is as much a "wiring adapter", as it is an actual performant electrical thingy. The impedance transformation is seldom perfect.

People who *transmit* really care about stuff like this, which is why they joke about "SWAR grease" (Standing Wave Ratio grease). There's no need for jokes about SWAR grease with TV/FM antenna bodges. It's all low power and carefree, for the most part. Only a dead short would hurt :-)

"The spoof SWAR-grease advert in the 2017 april edition of RadCom made me smile. Many years ago as a teenager there was a joke doing the rounds about a special paint which would improve the SWR of your aerial."

Paul

[Martin Brown]

The loss of performance on a receiver with slightly mismatched impedance antenna feed is surprisingly small in practice. Main effect is to be less frequency selective and a little less gain.

Transmitting the mismatch would hurt efficiency but modern receiver front ends really don't care that much about exact impedance matching any more. I have a folded dipole in the loft (and a set of now unused reflector elements since it was a full yagi originally).

Folded dipole with the right dimensions should be close enough:

[Graham Nye]

(For OP) Presumably you measured the resistance with a multimeter. This just measures resistance using DC. It can't measure impedance at a radio frequency. You need fancier equipment to do that.

[Andrew]

Agreed, but there is a dead short between central pin and shield on the aerial that came with the DRA-455, while the similar aerial that came with my Denon DM41-DAB has an infinite (DC) resistance between pin and shield.

What I don't know is if there are any components inside the actual coax plug because it has a longer, tubular black plastic body that normal TV coax plugs don't have.

It's not an issue, I have a 3 element yagi in the loft which gives an acceptable signal for the DM41, I just need to add a Y connector and run some more coax down into the lounge parallel to the TV downlead (the DM41 is upstairs in my 'office').

[Paul]

There are some pictures here.

Paul

[Graham Nye]

If you have an aerial made from 300 ? ribbon cable, i.e. a pair of parallel wires like this -

you would expect it to read under 1 ? on a multimeter. The wires form a continuous loop around the feeder and antenna from centre pin back to the shield.

If you have an aerial made from single strand of wire, like this -

you would expect to read an open-circuit (i.e. infinite resistance). The wires connected to the centre pin and shield are not connected to each other.

(PS Don't buy this one; it's a ridiculous price. I'm only quoting it for the picture.)

[Martin Brown]

One is likely an open dipole which is an open circuit at DC, and the other is a folded dipole which at DC is a short circuit. Simple open dipole ___ ___ ||

Folded dipole (often made out of spaced parallel tramline wire.

________ |___ ___| ||

There might be an impedance matching balun in it then or a small ferrite transformer.

Passive splitting the signal will also cause an impedance mismatch although chances are with a decent antenna you will get away with it.

[Andrew]

It's like the 2nd link. A pair of parallel wires exitting from the coax plug as far as the plastic dimple, then just single wires intended to be spread horizontally. And a zero ohm resistance between these two individual wires. I think it is faulty. I have been up in the loft today running some more coax down to the lounge for a proper signal from my 3-element yagi

[ARW]

Slightly aside to your question.

I called in to see friends a couple of weeks ago in Doncaster who required an aerial for a Sherwood TX-5030C (which has two selectable FM inputs).

So I asked Bill Wright for help and he made me one out of a small length of cable (I got to choose black or white cable) connected only to the inner pin of a coax connector.

Holme Moss to that part of Doncaster (trust me Bill knows everything) has a good signal and it worked perfectly. ISTR Bill said keep it vertical or if horizontal at 90deg to the transmitter but it works when coiled up:-)

There are some advantages of going to the pub with Bill.

[Andrew]

After a recent national re-tuning exercise I went up in the loft and removed the old Group C/D 11 element aerial leaving the builders coax lying looped up on the loft insulation.

To my amazement, the TV downstairs was still showing BBC1 in HD with a perfect picture and sound. None of the other channels would work, so the builders coax alone was a perfect aerial for certain frequencies.

I visited my sister in Barry last week and slept in the top bedroom overlooking the Bristol Channel. The TV up there just had one of those set top aerials that looks like a hand-held fan. This picked up all the Mendip channels perfectly.

[williamwright]

There are many advantages. Here are just two: My amazing wit and repartee It makes you seem relatively good looking.

Bill

[Sn!pe]

In the context of the [snipped material]:- I would have just stuffed a ~1.5m length of wet string in the 'ole and expected a similar result. ?:o))

--g8dgc

[Sn!pe]

Bother! I meant 0.75m of course. [red face]

[ARW]

And you paid:-)

[tony sayer]

Indeed there are!.

Bit here about some very tough Yorkshire men who built the old moss!.

And as Mr Double n barrelled whatsisname says "Bah gum is bloody cold y up here" and he was right abat that!..

And b from BICC back in 1952 ish!...

[ARW]

We only needed Radio 3 and 4 so the wet string could be tuned to better than 0.75m:-)