H Line Observation


The activity took place under the initiative of my friend Ferruccio IW1DTU.
He suggested that my 10m dish, used for earth-moon-earth activity on 1296Mhz, could be used to do some astral object observations on the neutral hydrogen line at 1420.4 Mhz.
We, both, dedicated one day ( 3rd/Jan/2014) trying to identify the visible sources of radio frequency.

Set Up

The setup was composed by the following:

I1NDP Installation

10m dish
1296Mhz feeder

IW1DTU portable receiving station

Professional receiver spanning from HF to 4Ghz
SDR 14 connected to the 10.7 Mhz IF output of the receiver
Sprectravue software
Personal computer

Sun and Moon

We started with the observation of well known radio frequency source mainly to check the behavior of the antenna-feeder combination used on a different frequency
They are observed in the time domain by moving the antenna above the astral object.


Sun noise was much lower than expected, compared with the same reading on 1296Mhz shows a loss in sensitivity of about 6db.
The conclusion in that the system performs more or less like a 5m dish having the proper feeder and LNA for 1.4Ghz. In any case still a good antenna for such observation.


Moon noise was taken at the native frequency of 1296Mhz and shows nearly 1db of excess noise


Neutral hydrogen line at 1420.4 Mhz

We continued trying to identify some of the most interesting and visible source of Radio emissions but starting from a cold sky region to be used as reference for the following observations.
The hydrogen line is observed in the frequency domain.

Acquarius cold spot

Despite the quiet region, still a well visible hydrogen emission

Milky way (10 degrees longitude)

A well visible peak of H emission

Same as above (bidimensional)
an annoying spurious signal present at 1420.75 Mhz


Milky way (110 degrees longitude)

3 peaks of radio emission showing 3 different blocks of Doppler effect.
The 1st with a frequency lower than the nominal 1420.4 Mhz is receding while the other 2 are approaching. The emissions are generated by different milky way arms

Same as above (bidimensional)


Sagittarius A)

Located at the center of the milky way (at a distance of about 25,000 light years from us) is a powerful source of radio emission.

Same as above (bidimensional)
Two blocks of radio sources both approaching but at different speed



The constellation is showing a beautiful shape

Same as above (bidimensional)
Two blocks of radio sources the one at the left is receding while the right is approaching



The constellation is receding at time of observation

Same as above (bidimensional)


Off Line Data Processing

The above observations were all done taking advantage of the built in capabilities of an SDR14 receiver and the pictures were taken as screen shot of the SDR graphic interface.
I recently (Jan/2015) bought an Airspy SDR covering up to 1.8Ghz with 10Mhz bandwidth which can be used with the freely available SDR sharp software but with no provisions for radio astronomy purposes.
I decided to give a try by doing short recordings at 1420.4 Mhz and try to pull out of them the H line signals pointing the antenna to visible radio sources.
The off line process consist in passing the time domain recording though an FFT an then integrating several blocks of data in the frequency domain.
The results are in accordance with the previous observations:

Sagittarius A)

Even in this observation the whole shape is approaching but at different speed.
The signal is spread over 300Khz of bandwidth and, knowing the frequency and the Doppler shift, it's easy to calculate a maximum approaching speed of about 63 Km/sec.


Cygnus A)

Cygnus A (3C 405) is among the strongest radio sources in the sky at about 600 Mly far.
The galaxy was not visible during the previous observation.


Another view of our galaxy with the antenna beaming to the Perseus arm
The hydrogen is emitted from an approaching arm of the milky way.
A couple of spurious signals have nothing to do with the observation.

Milky Way 70°)

The whole milky way can be mapped on the base of its Hydrogen emission though not from the same earth location .

It can be done by pointing to the several constellations populating the galaxy or (better) using the astral coordinates , yet another view of our galaxy at 70 degrees longitude.

Two different arms visible in the shape.


  1. The graphic presentation is certainly not as fancy as that of SDR14 being the result of a fast home made effort. Apologies for that.

  2. You may notice different shapes for Sagittarius and Taurus signal comparing the two observations but they were made at different time and hence a different relative positions with earth.

  3. Signal of Taurus & Sagittarius were clearly visible also in real time on the receiver (so with no integration) although with a much worse definition.

Yet another Hline view

Recorded with my B200mini utility July-2021 covering the visible milky way from norther hemisphere at summer time

Single View at 150 degrees

The higher H line peak is approaching at 50 Km/sec

The Following is collecting different recordings each at 10 degrees distance: