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INTERFERENCE/ REJECTION FILTERS
😀 PART15 is back! Great!!!!
Question, where would I find specific info on interference filter design or notes on rejecting frequencies after 1200khz… I am broadcasting at 1200khz and hear the signal on 6600, and on others as well. Is it because my radio is close to the transmitter or is the air coil transmitting harmonics beyond the 1200khz primary. What can I do to solve this. I heard of the 7 pole and higher filters but I need a specific design for 1200khz. Has anyone look at the frequencies beyond what their primary frequency of broadcast is? How do you know you’re not transmitting in the multiples of harmonics?
Total posts : 453666600 is not an integer multiple of 1200. Harmonics are integer multiples, but you may have actually been tuned to 6000 or 7200. If your receiver is very close to the antenna, you can get all sorts of spurious signals at weird frequencies, usually due to receiver overload at the transmitter frequency.
If you are using a tuned antenna (some sort of loading coil and whip antenna) it will have a fairly sharp tuning peak. In radio jargon, the antenna will have a “high Q”. This is a very good thing to have for rejecting harmonic radiation. Your antenna will radiate very well at the tuned frequency and very poorly at harmonic frequencies.
Make sure your antenna is tuned precisely to your frequency for best performance. Even with a high Q antenna, you will still get some very low level harmonics that you will hear on a nearby receiver. My bet is you are too close to the antenna, and moving the receiver away a couple dozen feet will make the 6600 signal all but disappear.
There are “low pass filters” that will greatly attenuate harmonics, but they are typically designed to go in-line in a 50 ohm coax feed. Even if you are using coax, I don’t think such a filter is a good idea because it will add some loss to an already anemic 100mw signal.
Phil B
Total posts : 45366I’d say PhilB makes a good point, the harmonics from a 100 mW signal are likely less than smothering 😆
However, one on my nextdoor neighbors did chew me out last year for splattering all over CB channel 4, so I guess it can come up. I was using an old chopper XMTR at that time, and those do send out the hash.
I switched to a PLL and CB channel 4 is safe now (whee!), but if a person was still interested in filters, the old MWA site has some study plans here:
[url]http://www.geocities.com/raiu_harrison/mwa/tech/circuits/filters.html[/url]
Enjoy 8)
Total posts : 45366[url]http://www.geocities.com/raiu_harrison/mwa/tech/circuits/filters.html [/url]
That information looks good. But, look closely, “Unless otherwise noted these filters have 50 or 52 ohms input and output impedance.”
Where you put such a filter in your circuit is important. You need to know the impedance at the point you insert the filter. If you have a Rangemaster, for example, you can’t insert the filter between the transmitter and the whip antenna because this is a HIGH impedance point. You would need to insert it internally inside the transmitter before the tuning coil. This is impractical and may not work without knowing the exact impedance at that internal point.
I highly recommend not nobody wastes their time fooling around with such a low pass filter unless they fully understand the underlying principles.
Phil B
Total posts : 45366I finally remembered my password so I could get back in here. I tried to get a new one, but I could never get the automatic mailing. Anyhow…
Say, Phil… Doesn’t that pi net tuning in the SStran act as a low pass filter when coupled to the ten foot wire? Or is the impedance too high to be effective?
Or have I been away from the nuts and bolts part of radio too long to know what I’m talking about? 🙂
Bill in SE Texas
Total posts : 45366Actually, it acts as a very sharply peaked tuning circuit. A pi-section acts as a low pass filter when the input and output capacitances are equal and the output load is purely resisitve.
In this transmitter, the output load is purely capacitive reactance. The output trimmer capacitor is in parallel with the antenna for tuning. The input capacitor sets the load presented to the RF output transistor and the series inductor(s) are selected to resonate with the antenna/trimmer capacitance.
This is not a classic pi-network tank or pi-section low pass filter because the output load is not purely resistive.
When using the base loaded vertical antenna. The internal inductors are bypassed out of the circuit and the trimmer is connected in paralled with a fixed cap to ground. The result is an L-net formed by the capacitors and part of the external loading coil. The trimmer then serves as a loading adjustment and the antenna is tuned to resonance by selecting a coil tap and adjusting the length of the antenna. This is a vastly more efficient tuning method resulting in much longer range compared to the internal tuning circuit which is good for shorter range and convenient tuning over the entire band.
Phil B
Total posts : 45366Thanks fellows… I was out on travel…
I have placed the radio away from the transmitter and I have to say the harmonics are removed… seems that the reciever was too close! But the subject of cancelling ANY and ALL harmonics from other frequencies is very important to remove any interference to any licensed stations, especially the government ones since they can shut us down… I looked at the SSTRAN circuit and the pi-network is not a low pass filter, but a tunable circuit for the antenna matching… I agree that the LPF needs to be inserted before the pi-network… I will try several that are posted on MW Engineering websites… but will that void any circuit design by SSTRAN especially if the FCC opens the SSTRAN to see if any mods were made to the transmitter based on the circuit diagram/manual… they may think you are upping the wattage, etc. of the transmitter, etc.?????
Total posts : 45366When the SSTRAN transmitter is set up for use with the external base loaded antenna, the switches inside the transmitter are set to bypass the internal inductors which disables the internal tuning network. The base loaded antenna, when tuned properly, has a very low resistive impedance which gets lower and lower as you improve your ground system. The matching circuit formed by internal capacitors in the transmitter and the loading coil is an “L-net”. There is no point in the circuit where you can insert a 50 ohm low pass filter without severly disrupting the operation of the L-net.
Short base loaded antennas are a strange breed when compared to much bigger amateur and commercial antennas. The radiation resistance is only about 0.1 ohm compared to 50 ohms for a full size 1/4 wavelength vertical. Because the radiation resistance is so low, there is essentially nothing in the resonant antenna circuit to absorb power other than coil and ground losses. This results in a very high Q which means the tuning has a very sharp peak. The high Q causes the tuning to be critical, but also has the added benefit of greatly reducing any off-frequency spurious radiation like harmonics. A low pass filter is not necessary.
Even if you were able to add an effective and efficient low pass filter, you would still have the same problem with overload in a nearby receiver. The overload occurs because the level of the primary transmitted frequency is causing the transistors at the front end of the receiver to go into non-linear operation. You will still get this even if your transmitter has absolutely no off-frequency radiation.
Phil B
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