Hello, it has been some time since I posted here.
I have been building an SSTran Antenna.
Hello, it has been some time since I posted here.
I have been building an SSTran Antenna.
I asked this question before but with what I have read, I would like to ask it again. Will the SSTran Antenna work with a Ramsey AM25? This link says it wont work with a Ramsey if it is a basic unit designed to work with a wire antenna. I think the AM25 is designed to work with a coil also, is this true. I want to make sure this antenna will work with my model before I finally build it.
Next, what would be proper protection from lightning. I plan to install this at ground level. (I have little experience with this)
Finally, if I build it on ground level, what would be the best size for radials. I was planning on 10x10. Is this a good size. I was planning on using 16 of them. I know I've heard the more the better, could someone tell me the best amount for a yard about 60x30?
Thank you everyone!
I'll give some information regarding your first question. I checked the link and read it differently than you apparently did. It says:
ANTENNA FOR SSTRAN AMT 3000 AND RAMSEY TRANSMITTERS
(or for any transmitter configured like it, excluding the Rangemaster and transmitters configured like it)
The author is not excepting the AM-25, rather the Rangemaster. This is because the Rangemaster (and I believe also the Procaster) has an internal antenna loading coil. The AM-25 does not.
Though I have not tried it, it seems the SSTRAN antenna design will work with the AM-25. My reasoning follows, and forgive me if I repeat some things you already may know:
The purpose of the base loading coil is to provide inductance which cancels the high capacitive reactance of a 3 m antenna at resonance so the transmitter sees only radiation resistance plus the ground and coil resistance at the antenna feedpoint. This maximizes the antenna current and thus the radiated signal. From simulation data the expected feedpoint resistance for a ground mounted base coil loaded resonant antenna with radials is in the range of 20 to 100 ohms. Others have suggested using 12 to 16 10 foot long radials, a compromise between hardware required and ideal performance.
I have bench tested my AM-25 using various dummy resistive loads and found the maximum output power occurs when the load resistance is 24 ohms. Using this as 100%, the power delivered to a 101 ohm resistor is 57% and to a 11 ohm resistor, 81%. So, the AM-25 tolerates a resistive load range comparable to that expected from a resonant base loaded antenna and should satisfactorily work with this antenna.
The SSTRAN transmitter is a bit different from this. It needs to see a bit of inductance in the antenna system so the low antenna resistance can be transformed up to about 750 ohms. So the antenna needs to be tuned slightly off resonance when used with the SSTRAN yet, according to my tests, it needs to resonate with the AM-25. For this reason, I would add a few more turns to the coil and more taps to allow a wider adjustment range to achieve resonance. The best way to tune the system will be to use a field strength measurement.
I would be comfortable with building this antenna to use with the AM-25.
Neil
Hello
I agree entirely with Neil that a loading coil improves AM25 but that a different number of windings is needed because of the difference in impedence compared to SSTran.
I had very poor results using only a 3-meter vertical, but found that the right amount of turns around (in my case an oatmeal box) a cylinder puts the antenna into resonance and the radiation does well.
I have yet to experiment with grounding of my transmitters so I can't share any experience, but I have a hunch I'd like to try sometime, that longer radials in the direction(s) you most want to cover might improve the signal in that direction. Again a guess, I would think going to a length multiple of the true wavelength of your frequency would reach the best result. Neil?
Some comments on your question regarding lightning protection. This is a very complicated subject but here are the fundamentals:
Protective ground: The best way to do this is to establish a single ground point for everything in a building and an antenna structure. Usually, this is done by bonding the antenna ground to the ground stake used for the electrical service. The reason is that if a direct lightning strike occurs there will be a potential gradient of thousands of volts per meter along the earth near the strike. A ground stake at an antenna say 20 meters from a building ground stake will be at 20,000 volts potential difference from the building ground. It is virtually impossible to protect electronics from a direct strike but some things can be done for nearby strike protection.
DC bleeder: With nearby strikes and even when the wind blows, an antenna can become charged. It is a good idea to provide a DC path from the antenna to ground. Two ways to do this are to use a choke inductor of high enough impedance at the operating frequency from antenna to ground to provide a DC path. The other way is to use a "bleeder" resistor of say 1000 ohms from the antenna to ground. Neither will affect the RF performance. One could also use a gas discharge tube such as a NE-2 neon bulb to clip the induced voltage spikes. Not very good but better than nothing. Note that this only provides minimal protection from a nearby strike but it is easy to do.
Now the most important part of this.
PROTECT THE BUILDING AND ITS CONTENTS. This is the primary thing since property damage and life are at risk and we must do all we can to keep the lightning energy out of the building. Every conductor from your transmitter and antenna entering the building needs to be protected with a spark gap or similar device which is attached to the building ground as close as possible to the point of entry. This is required here by building codes and is an important thing to pay attention to. Even if the transmitter at the antenna base is fried the building and its contents, including you, will be protected. There are various devices which can be use but since the operating voltages are low, I would suggest the inexpensive cable TV "ground blocks" which attach with F connectors.
I have offered only general guidance here and since situations are different I cannot assure that this advice is complete nor adequate but it does, I trust, point you in the right direction. You might search for ARRL articles on this topic.
At the very least you need to realize that you need protection where the wires enter the building.
Neil