Antenna tuner/ground plate

[Paul Elliott]

Originally Posted by Seeking Solace

How many TV's, scanners, car radios, have resonant antennas? I'd say nearly all of them. Go slap a tape measure on your car radio's antenna I bet you'll find that it closely meets the correct wavelength for it's intended bands. It's not a perfectly resonant antenna, but it doesn't have to be exactly perfect.

Since the AM radio band runs from 0.54 to 1.6 MHz (a 3:1 frequency range), and a car's whip antenna is extremely short (about 1/300 of a wavelength), I don't see how that whip antenna can be resonant. It's not. The whip antenna uses (or used to use) a low-loss high-impedance shielded cable, but the antenna impedance is essentially a very low resistance in the milliohm range with a large shunt capacitance. This is definitely not resonant.

Again, I'm not claiming that resonant antennas are bad. In fact, I prefer them. But there are plenty of examples of non-resonant antennas, and your car radio antenna is one of them.

[Seeking Solace]

Afternoon Paul

The antenna on your car is really for the FM side of the radio. It does work, although with poor efficiency on AM. Ever notice how lousy the AM side is compared to the FM? There's a lot of factors at play there, mostly that FM stations are usually more powerful. They also have a much smaller coverage area because of frequency.

The average FM radio is 87.5 - 108 MHZ. The middle of the broadcast FM band would be about 98 MHZ (no this isn't exact). So how long is your average FM antenna on a car? It's 38 inches.

Let's do the math. 468 / F Mhz = 1/2 wave 468/ 97 = 4.7755102004081632 feet. Now we convert the feet to inches 4.7755102004081632 / 12 =0.39795 = 39 inches.

I'd say that is pretty darn close to being a resonant antenna. A resonant AM antenna wouldn't be practical. The designers know this and I'm sure they compensate with an LC circuit in the radio for the much shorter antenna on that band.

[Paul Elliott]

Yes, of course the car antenna is roughly a quarter wave whip for the FM band. But that's not what I'm talking about. I am trying to point out that using your car antenna example, we can see that a very short, non-resonant antenna can work adequately for reception. And that antenna is several orders of magnitude electrically shorter than the untuned wires we are suggesting for marine HF use.

I'm going to stop beating this horse for a while. You and I are both choosing extreme cases to make our points, and by this time I doubt if anyone else cares. I'm pretty sure that we could both compromise somewhere in the middle and be satisfied with the results.

[Seeking Solace]

Agreed. I just hope the original Op got his rig working. I did enjoy the exchange. A worthwhile debate. Perhaps it will inspire someone else to come to their own conclusions. Thanks for yours and others input.

I even respect John's input. He's clearly knowledgeable.

[transmitterdan]

Solace,

Some math problems there. The 1/4 wavelength at 97.5MHz (geometric mean of the FM band in the US) is about 30 inches. That's about what vertical whips on cars are set at. The "impedance" of such an antenna is about 70 ohms. Even so it works ok. A half wave antenna has a very high impedance (near infinite) and generally folks try to avoid that.

On the AM band a 30 inch vertical whip antenna is electrically short so its impedance is no where near 50 ohms. Typically it will be on the order of 1 ohm. An L/C tuner capable of matching the 1 ohm antenna resistance to 50 ohms will have more losses than it can make up for unless it is very big and bulky which car radios cannot afford. Plus it will have very low bandwidth and require retuning across the AM band. Little or nothing would be gained by having such a matcher. Better to just let the antenna feed the 50 ohm coax input directly. With or without an L/C matcher there will be no standing wave on the coax between the antenna and the receiver. Power is flowing from the antenna into the receive port. So long as the receiver input is impedance matched to the coax impedance there will be no reflected signal out of the receiver and thus no standing waves on the coax.

[Seeking Solace]

I've no idea what the impedance requirement is for an FM receiver in a car. I've never bothered to measure it and won't assume it's supposed to be 50 ohms. My 40 meter antenna is fed with 300 ohm twin lead, only the balun is showing 50 ohms to the transmitter and changing the angle the antenna is oriented away from 45 degrees rapidly changes it's feed point impedance.

In the end as long as you've got more signal than trash you should be able to demodulate it. Doesn't mean it's going to be ideal or even perfect. Even a poor reception is better than no reception.

Like the other OP said, we've beat this one in the ground. I've presented my best educated opinion about the importance of resonant antennas. If someone is happy with a non resonant antennas performance so be it. My experience is based on many years of working stations around the globe. If I tune my antenna I can hear much weaker stations.

I hope that people will decide for themselves by trying both and maybe learn a little something about how RF works in both directions.

[osprey877]

I am still not getting any signal, just noise. I traced the end of the antenna cord (not easy because there was a hidden splice to different coloured and sized coaxial) to a "NASA Marine HF Antenna". Pictures attached ( The white sleeve with the writing on it in the first photo fits over the antenna board in the second photo to protect it from the elements. I just removed it for the purposes of taking the photos). I checked both the centre conductor and the shield for continuity between the antenna and the coax connector on the back of the radio, so that seems okay.
Disconnecting the antenna from the radio while turned on results in a slight but noticeable decrease in volume.
I also tried the radio while out on the water at least five miles from any marina, but stlll nothing but noise.
What is a good test station or frequency I should be able to hear? I have tried 5, 10, 15, 20 and 25mhz for the time signal, but just get noise. The signal strength metre on the radio says I am getting a strong signal, but it is all just noise.
Any suggestiions?

[Seeking Solace]

Originally Posted by osprey877

I am still not getting any signal, just noise. I traced the end of the antenna cord (not easy because there was a hidden splice to different coloured and sized coaxial) to a "NASA Marine HF Antenna". Pictures attached ( The white sleeve with the writing on it in the first photo fits over the antenna board in the second photo to protect it from the elements. I just removed it for the purposes of taking the photos). I checked both the centre conductor and the shield for continuity between the antenna and the coax connector on the back of the radio, so that seems okay.
Disconnecting the antenna from the radio while turned on results in a slight but noticeable decrease in volume.
I also tried the radio while out on the water at least five miles from any marina, but stlll nothing but noise.
What is a good test station or frequency I should be able to hear? I have tried 5, 10, 15, 20 and 25mhz for the time signal, but just get noise. The signal strength metre on the radio says I am getting a strong signal, but it is all just noise.
Any suggestiions?

What is your Geographic Location? I'll post in a separate post a suggestion of time and place. Also, when you say just noise are you saying static or are you hearing the audio from the weather fax?

[Seeking Solace]

I use Sea TTY for weather fax. It's not to say that it is any better than any other program to decode the signal.









As you can see the weather is an image based system that uses FAX. If you're listening for a voice report you may be missing it because you're not on schedule with or on frequency with a voice signal.

Also, signals are 1.9 khz down from the reported frequency on USB. You can get the full schedule from the NOAA website HERE as PDF

It does sound pretty annoying when it's sending endless fax files. If you didn't know what you were listening for you might even think that it was just noise.

[Seeking Solace]

Something I forgot to mention that I just thought about. All of those times for broadcasts are in UTC time. So you'll have to adjust your time for the offset. Also, higher frequencies during daylight hours and lower frequencies at night. Although if close enough to any of them you're likely to hear them anytime they are being sent.

I'm a 3 hour drive from New Orleans and can hear the 4.316 Mhz day and night with it's 4KW transmitter monster signal. It's better at night, but I still get an average meter reading of S7-S9 during the day and 20+ at night. My rig has a dedicated output for digital modes and doesn't overdrive my laptop's input even at full.

[Paul Elliott]

I suggest that you eliminate the active antenna (which could be fried or have no power), and eliminate the existing cables and connectors. Connect an insulated wire, any gauge, from the radio antenna connector, using (as has been suggested) a bent paper clip, or by just folding back the stripped end of your wire. Plug this into the center of the radio connector, and don't worry about the ground.

Run your wire out through a port hole, or a hatch, or the companionway, and secure the end up six feet or so in the air. Tie it to a shroud if you like.

Now turn on the radio, and tune to the frequencies for WWV (5, 10, 15 MHz) and see what you hear. If you don't hear anything, unplug the antenna wire and see if the noise drops. If it does, the antenna is probably doing a decent job and you need to keep trying.

As you may have noticed, there is nothing complicated or precise about this. The goal is to see if your radio works. Once you get that figured out then you can work on improving the antenna.

[Seeking Solace]

Paul's got a good line of thought going there. Also according the antenna manufacturer.

Note – if your HF3 receiver has a red couloured label on the rear chassis the power supply for the antenna is already built in. It is activated by the slide switch on the rear.

Your has the red label according to the pictures you sent. Doesn't mean it works or that the antenna isn't trash. Doing what Paul said is a sure fire way to know if the receiver is working.

[osprey877]

Originally Posted by Paul Elliott

I suggest that you eliminate the active antenna (which could be fried or have no power), and eliminate the existing cables and connectors. Connect an insulated wire, any gauge, from the radio antenna connector, using (as has been suggested) a bent paper clip, or by just folding back the stripped end of your wire. Plug this into the center of the radio connector, and don't worry about the ground.

Run your wire out through a port hole, or a hatch, or the companionway, and secure the end up six feet or so in the air. Tie it to a shroud if you like.

Now turn on the radio, and tune to the frequencies for WWV (5, 10, 15 MHz) and see what you hear. If you don't hear anything, unplug the antenna wire and see if the noise drops. If it does, the antenna is probably doing a decent job and you need to keep trying.

As you may have noticed, there is nothing complicated or precise about this. The goal is to see if your radio works. Once you get that figured out then you can work on improving the antenna.

What should I hear? voice, morse code or fax?? I'm not at the boat now, so I can't test.

[osprey877]

Originally Posted by Seeking Solace

What is your Geographic Location? I'll post in a separate post a suggestion of time and place. Also, when you say just noise are you saying static or are you hearing the audio from the weather fax?

Blaine, Washiington.

[alctel]

Originally Posted by Seeking Solace

Paul's got a good line of thought going there. Also according the antenna manufacturer.



Your has the red label according to the pictures you sent. Doesn't mean it works or that the antenna isn't trash. Doing what Paul said is a sure fire way to know if the receiver is working.

stupid question - as it has an on switch, has it been turned on