RadioBanter - View Single Post - Newb Question

Telamon · September 25th 06, 12:46 AM posted to rec.radio.shortwave

In article ,
(Mark Zenier) wrote:

In article

, Telamon wrote:

A better antenna in the same space would be a folded dipole, which
would use the common and cheap 4:1 transform used with FM and over
the air TV antennas.

The set is nowhere near a 50-75 ohm input. Especially if the RF gain
isn't fully on.

Looking at the schematic of the DX-160, the antenna inputs feed a 2k
ohm potentiometer, (the RF gain control), (with a back to back pair
of protection diodes from the wiper to ground), and then are switched
to the input windings of the first tuned circuit.

Next, there's a cascode FET RF amplifier and another tuned circuit
feeding the mixer. Both tuned circuits are tuned with the main and
bandspread variable capacitors. And the input coil is tuned with an
antenna trim control, too. (There's plenty of places to mis-tune
this RF amp, if the unit seems a little deaf). Then to a mixer and a
455(?) kHz ceramic IF filter.

I would try a folded dipole direct to the balanced A1-A2 inputs. The
set doesn't need a balun.

Or Google up RHF's posts from a year or three ago about using a room
sized loop on a ceiling or wall with the DX-160.

The radio input impedance is a separate issue.

Re-quoting RHF post I specifically object to the following.

* Use a Balun with a Dipole Antenna Another 'improved' Shortwave
Listening (SWL) Antenna design would use a Balun {Matching
Transformer} between the Dipole Antennaand the Coax Cable
feed-in-line. This is most often a 4:1 {Four-to-One} Matching
Transformer. Using the Balun will better 'match' the Dipole Antenna
to the Coax Cable feed-in-line and the Receiver across a wide
frequency range then simply using a Dipole Antenna that is directly
connected to the Coax Cable.
- - - The direct conneted Dipole Antenna is great for Hams
working on a single Shortwave (HF) Frequency Band.

RHF wrote about matching a dipole.
I wrote about matching a folded dipole in a follow up to his post.
A dipole is around 72 ohms and a folded dipole is around 300 ohms.

********

There are three items involved, antenna, transmission line, and radio
input impedance. They all have to match. The 4:1 BALUN is incorrect to
match a dipole to 50 ohm coax what is happening on the other end of the
coax does not change this relationship. The closest easily attainable
would be a 1:1 BALUN. All impedances need to match or you have
reflections and the signal that the antenna picks up will only
partially be used by the radio input.

If the 50 ohm coax to radio input is an issue then you need another
transform between the coax and radio input. Most radios have a 50 ohm
input. Generally, you can figure this out by the connector used. On
most radios the radio input may not be right at 50 ohms but will be
somewhere in that area over the 3 to 30 MHz input range.

--
Telamon
Ventura, California