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High performance filter projects for the 145MHz amateur band.
pa0nhc 2002 07 18.   Corr: 20110307  20150220
Site location: www.pa0nhc.nl

Many Many listeners on the two meters band are often hindered by loud noises, caused by inter modulation from their own receiver, due to the intrusion of very strong out of band signals, from data networks (150-165MHz) or FM-broadcast (88-108MHz).

In the articles below you will find construction descriptions for cheap home made filters against such IMD.

The coaxial notch filter is easy to make by every radio amateur.

For the construction of the other filters you must perform some simple mechanical work, and be able to measure VSWR and insertion loss using low power signals.

Advise:

On the two meters band, a 2 resonator helix band pass filter gives minimal insertion loss, maximal selectivity, and a silent background. You can build it at very low cost with simple tools.

For still steeper filter flanks and better suppression of signals very close to our two meter band, simply add one 155MHz-cable notch, with little increase of insertion loss.

A simpler wire-coupled helix band pass filter is the best choice to attenuate signals below 145 MHz (FM-broadcast and TV-band 1).

 

The drawings and filter characteristics on these pages will be downloaded in a high resolution, but resized to smaller dimensions, to fit your screen.
Drawings can be printed on paper in optimal quality.

Single helix resonator
145MHZ band pass filter.

Two helix resonators
more selective 145MHZ band pass filter
Bandpass / reject combination
for 145MHZ. Super selective + maximal suppression of 155MHz.
Two left hands ?
Coax notch filter
for 145MHZ.
Very simple to make.
Single helix resonator
145MHZ band pass filter,
with optimized suppression of FM- and TV broadcast.
Four helix resonators
  Suppression filters
for 155 to 170MHz.

Helix NOTCH filters

Here some pictures from the radio spectrum below 200 MHz,
As seen in 2001 on my home build spectrum "analyzer", and received on my 2m/70cm dual band antenna at 45 meters height, 8km west of Rotterdam city center.
Observe that some signals from pager networks in fig. 1, are 30 times (!!) stronger than our already very strong local repeater pi3rtd in fig. 4.
The signals at 145-146MHz in figs.1-4 came from different relay-stations, and consequently are different in strength.

Scales: 20 MHz/div. and 10 dB/div. Baseline = 1uV = S7.  Left=0 MHz, mid=100 MHz, right=200 MHz.
(20dB less means 10 times weaker, 40dB less means 100 times weaker, 60dB less means 1000 times weaker).
In yellow: signal strengths according to European amateur-norm: S9 = 5uV over 50 ohms,
1Spoint = 6 dB.

In fig.1 the spectrum, as received on my 145/435 MHz dual band antenna at 45 meter height 10km east of Rotterdam center, without filtering.

The pager networks ("Data") sometimes peak to S9 + 55dB.

Such strong signals will overload the mixer stage of many receivers, causing loud noises (IMD).

TV = S9 +35dB, FM = S9 + 45 dB. 

 

Fig.1, received spectrum without any filtering.

 

In fig.2 the spectrum, as measured through the 3 notch helix notch filter. The filter was designed to suppress only 155-165 MHz. Compared with fig.1, the two signals in this band portion are suppressed enormously.

FM broadcast and TV channel 4 are only a bit weaker.

But, most other signals are nearly as strong as before, and still could cause noises.

Fig.2, spectrum through the 3-notch coax cable helix filter.

 

In fig.3 the spectrum, measured through one big size (18 cm diameter prototype), 145 MHz helix band pass filter. Now all signals outside the two meter band are attenuated.

The data-signals above 155 MHz are at least 20 dB attenuated.
FM broadcast is 35 dB weaker.
TV channel 4 is 40 dB attenuated.

Fig.3, spectrum through the single resonator helix band pass filter 145 MHz.

 

In fig.4 the spectrum, measured through a two resonator helix band pass filter for 145 MHz of 10 cm diameter. All signals outside two meters are attenuated strongly.

The strong 145 MHz-signal is our local repeater pi3rtd, 1.5 Werp at about 10 km distance.

TV channel 4 is attenuated 40 dB. FM-broadcast by 30 dB.

The data signals above 155 MHz showed to be attenuated 30 to 55 dB.

Fig.4, spectrum through the two resonator helix band pass filter.