c51. TVI filters

A. Antenna technik
B Amateur radio rigs and mods
C. Amateur radio experiments
D. VHF/UHF/SHF experiments
M. Measuring equipment

I was quite surprised to learn that Johnson already used the disc capacitors as shown in an advertisment in QST oct 52.

This note was presented in Radcom Technical topics March 1982, but couldn't be published in QST because they are dependent upon their advertizers and therefore must adjust the "truth" to match optimum income for ARRL. My note was therefore denied with an explanation saying that the two filters I had tested should possibly have been faulty.



Fig. 1. The Drake TV3300LP modified to form an absorbtive or so-called "hybrid" filter.
The added HPF is based on calculation. L6=L7=0.13µH (5t 10mm ID 18SWG).
C6=C8= 61pF calculated, and 68pF used. C7=30pF calculated, and 33pF used.
0.1dB ripple, 40MHz Fc, 50 ohm impedance chebychev type filter

Absorbtive low-pass filters for HF (Drake TV3300LP).

In November 1968 two Collins Radio engineers, wrting in QST introduced the concept of absorbtive low-pass filters in which the unwanted high-frequency energy is separated from the wanted output and then safely dissipated in matched resistors. This technique was later endorsed by Hans Rohrbacher, DJ2NN in CQ-DL August 76, and his design appears also in recent editions of ART. However, it has to be said that the majority of filters continue to be of orthodox design, in which the transmitter 'sees' a high or low reactance at high frequencies and which assumes that the transmitter output impedance at these frequencies is similar to that of the operating frequency.
In 1981 LA8AK, in conjunction with LA1VC, has been checking whether in fact absorbtive filtering does offer any substantial improvement. Some Drake TV-3300-LP filters were modified, using worst case components (ie ceramic rather than silvered mica capacitors, and standard value rather than non-inductive low tolerance values) and carefully checked the attenuation and and input VSWR (fig 4). It can be argued that the output of a signal generator alone, unlike a transmitter, will (or should) have the same output impedance at all frequencies and therefore may tend to underestimate the advantage of the absorbtive approach.
Nevertheless the results are very convincing. Insertion loss and passband vswr of the modified filter were found very similar to the unmodified original filter, and minor differences may be accounted for by manufacturing tolerance; the input vswr over the stopband, however, was very different (table). While the apparent fall-off attenuation of the unmodified filter may appear better , in practice this is likely to be lost due to high input vswr. Although the filters show laboratory attenuation over 80dB, it is probably that in practice 60dB may prove about the limit since the transmitter chassis will always contribute to some radiation.
It is considered to be important that the transmitter should not 'see' notch filters connected in parallel with the signal, as in some earlier absorbtive filter constructions. While it is difficult to determine all potential 'troublesome' parameters of an LPF, since different transmitters will be affected differently, it seems that the most important is the VSWR on the cable between the filter and transmitter. It is also critical that this cable should be short, as cable resonnances (which may short-circuit the output) will introduce problems - and indeed such techniques are often used to increase harmonic output of the doublers, triplers etc (idler circuits). In other words, the absorbtive (or 'hybrid) filter is very desirable beast.

Fig 2a). Insertion loss of original unmodified filter over the passband 2-30MHz
and between 30-42MHz (marker 33.16MHz) -14.4dBm.
b) Insertion loss of the modified lowpass-filter.

Fig 3a) Stop-band attenuation or the original unmodified filter
b) Stop-band of the modified filter.

Fig 4) Input VSWR of the "worst-case" modified filter over the range 30-240MHz.

Table 1. Measurements on Drake TV-3300-LP low-pass filters

2m notch filters [144MHz notch filters described in Radcom Apr. 82, technical topics, pp320]




Fig.1 Double-stub notch filters for 144MHz. A and C 17cm, D and F about 33cm (to be adjusted with twisted wire in the end), B 34cm (uncritical), E and G (used only to optimize filters for Band III TV) 12cm. Note that stub dimensions are affected by velocity factor (0.66) of the cable.

Fig. 2. Mechanical construction of the double-stub VHF notch filter. For practical reasons 60 ohm TV coax cable was used.

Fig. 3 Response curve of the three double-stub 144MHz notch filters tested. Filter 1 without channel optimization, filter 2 and 3 optimized for minimum attenuation on Channel 11 Band III, aproximately 220MHz.

Text may follow later

The VHF broadcast bands are:
Band I: TV Ch. 2-4 (DSB/VSB Video negative modulation, FM sound)
Band II: 87-108MHz FM sound Broadcast
Band III: TV Ch.5-11 (DSB/VSB Video negative modulation, FM sound)

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Last updated: 2004.03.01