When I first started investigating the theory of operation of the SSTRAN transmitter with the base loaded antenna, the explanations I received from several people did not seem credible. For the benefit of anyone else that might be cudgeling their brains over this issue, I recommend the following thread: www.part15.us/node/883
These gentlemen have done an excellent job of determining how this works, and I highly recommend reading their analysis if you are at all curious.
I would like to modify the base loaded antenna so that it presents a DC ground to help discharge static electricity. I live in a part of the country where lightning storms are very common in the summertime and I think having a direct connection to ground through the coil (e.g. by grounding the lower side and tapping up on it) would be helpful.
SSTRAN type antennas
WEAK,
It is good that you found the link you posted. I had forgotten about that. I wonder if Dave ever got it going on the air.
Regarding the lightning situation, as I think you know the idea is to bleed off static from the antenna to ground in the hope that this will reduce the probablility of a strike. A simple way to do this is to install a resistor from the bottom of the coil to ground. Another approach which requires a bit more care (because you are adding inductance) is to install a choke to ground.
The antenna I use here is an indoor antenna that I built for another application and modified for use with the SSTRAN. The loading coil is between a 3 m radiator and ground. A variable capacitor across this coil allows tuning for a peak signal. The transmitter signal is link coupled by a few turns around the grounded end of the coil. This gives a really low resistance to ground for the antenna and disconnects the transmitter from the antenna ground so I don’t have a ground loop.
This is just an idea in case you might want to do some experiments. I have no idea how this antenna compares performancewise with the SSTRAN design, but despite it being horizontal in the basement I can hear a fairly good signal mobile out to about 1000 feet or so.
Another research project here involves using a home made and home calibrated torroid transformer to measure the antenna feed current. Assuming a radiation resistance of .1 ohm from the measured current my radiated power is 108 microwatts. My next step is to measure the power into the antenna system but I am having a problem calculating the effect of the phase shift of the transformer which I need so I can calculate the power (VIcos(theta)). I measured this phase shift, but when I apply the correction for the measured angle between voltage and current I get an angle which doesn’t make sense so more work is needed before I trust the data. Thought you might have some interest in this.
Neil
Also very interesting
Neil,
I think your approach is very interesting and I will certainly give it a try. I thought about using one of my copier box antennas as the tunable coil like you described. I have built a number of these passive “H field multiplier” antennas for use as reception aids. They really do work phenomenally well! All I would need to do in this case would be to wind a couple of turns for the link and connect the antenna and ground wires. This is strictly an indoor antenna solution, but potentially useful to many, I would think.
WEAK-AM
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Don’t miss this
Don’t miss Phil B’s new post in the part15 us post you linked to (node 883) about the SSTRAN analysis. He presents some really good information for those who want to dig into how the tx. and antenna “play nice” together.
Neil