In practice it's not as clear cut as this antenna is better than that antenna. It depends where you are and what you're doing.
Even for AM broadcast, sometimes my simple small plastic loop from the electronics shop
performs better.

AM broadcast is vertical polarity. back in the day it might have done to point a horizontal
wire at your local station being directional, but switch to one from one to the other
might also bring in a lot of unwanted noise along with the signal.

The horizontal wire might also result in echo of propagated signals (fading in and out)
depending where you are. The best one is the one that works.

The wire I put up here was indented for HF, but is currently also my best AM broadcast antenna.
Not a dipole.. it's grounded from the feedpoint. Unfortunately I don't get to clear the house as much as I'd like.

 

My years of playing around with AM reception pretty much always shows that a tuned loop is smaller and much more convenient than a long wire with a good ground, and is entirely satisfactory for receiving the strongest stations, but an outdoor long wire with a good earth ground receives much more signal strength over a much wider range of frequencies.

So if you want to experiment with rf amplifiers, frequency conversion, detectors and audio amplifiers, I'd suggest making a small indoor loop tuned to your strongest local AM station and use it as your AM rf source.

But when you want to experiment all over the ranges from DC to 30 mHz, then an outside long wire and a good earth ground will pull in much more signal power.

So both antenna types have their place, in my opinion.

 

Thank you, MrChips, I would love to have an Enco Unimat. Am asking, but who's going to send me one? Will you?

 

Nice work, Art. What's the inductance of that big coil?

 

Your tip about going vertical is well-taken. I have an aluminum-sided mobile home so it makes sense from that point of view also. How long is your antenna?

 

My years of playing around with AM reception pretty much always shows that a tuned loop is smaller and much more convenient than a long wire with a good ground, and is entirely satisfactory for receiving the strongest stations, but an outdoor long wire with a good earth ground receives much more signal strength over a much wider range of frequencies.

So if you want to experiment with rf amplifiers, frequency conversion, detectors and audio amplifiers, I'd suggest making a small indoor loop tuned to your strongest local AM station and use it as your AM rf source.

But when you want to experiment all over the ranges from DC to 30 mHz, then an outside long wire and a good earth ground will pull in much more signal power.

So both antenna types have their place, in my opinion.

Your points are well-taken. After some experimenting I'll have a more definite point of view and maybe something to share, but right now I don't.

I understand that after he invented the radio, Marconi did a lot of experimentation with antennas. And as I understand it, that was back when transmission was a matter of getting a spark to jump a gap.

 

I've just rediscovered my interest in designing radios. I plan to start with an AM radio. I'm wondering about the antenna. In the past I used a long wire -- usually just a strand of magnetic wire strung over a distance. But I am now in a mobile home park and space is limited. I'm thinking of running a cable from my shop to the roof through a PVC pipe and fastening a wound ferrite rod at the top.

Does this sound like a good plan? Does anyone have a better one? After the AM radio I will be making a short wave receiver using the same IF strip and audio amplifier and hopefully, antenna. So this antenna must be versatile and sensitive.

You'll generally have more predictable results with a CLOSED LOOP antenna....they're much more immune to noise and don't require a ground. Look up the many FRAME LOOP antenna projects on the web.

Eric

 

Your tip about going vertical is well-taken. I have an aluminum-sided mobile home so it makes sense from that point of view also. How long is your antenna?

I would go with KL7Aj and try a tuned loop first and experiment from there,
the exception being, since you mentioned you're doing the radio yourself,
if it's something pre-superhet such as a TRF with minimal amplification,
then you would want to be going hell for leather with an outdoor wire and earth.

There is a list on the net of wire lengths to avoid if you look for wire antenna
lengths which is the half wave length of any frequency.

 

I have been using ferrite rod antenna's on the am band. I read an article sometime ago on that type of antenna. One would think they are horizontally polarized, but from what I recall from the article is that they are really vertically polarized when they are in the horizontal position. It seems the vertical magnetic radio waves are attracted to the ends of the rod.
I have also discovered that if you want to tune for example 600Kc to 1800Kc, you need a capacitor tuning ratio of the square of the frequency ratio. 1800Kc/600Kc = 3:1 so the capacitance ratio is 9:1 I also use a formula LC=25330/fxf.... where LC is inductance in uH,C is in pF and f is in MZ. This formula is a modification to the normal formula fr=1/2∏√LC.

 

your working with rather long antennas, anything very short is a compormise. with a wavelength of 500 meters for 600kHz, a half wave dipole is 250 METERS long. a quarterwave vertical is 125 METERS long. anything shorter for a vertical has to be inductive ly or capacitively loaded a lot, or it is a "voltage probe" antenna. either an air core loop or ferrite core loop would be realy good, and have the advantage of being able to rotate to null out interference from power lines and such.,

 

your working with rather long antennas, anything very short is a compormise. with a wavelength of 500 meters for 600kHz, a half wave dipole is 250 METERS long. a quarterwave vertical is 125 METERS long. anything shorter for a vertical has to be inductive ly or capacitively loaded a lot, or it is a "voltage probe" antenna. either an air core loop or ferrite core loop would be realy good, and have the advantage of being able to rotate to null out interference from power lines and such.,

I just finished building an AM radio. For an antenna I ran a small (about #32 magnetic) wire across 10 or 15 feet of my mobile home roof, and I'm getting all the AM broadcast stations in Yakima but with noise. The noise I hear from the speaker is like the rushing of the "snow" from an old TV when the station went off the air (high audio frequency, I suspect), and there is a 60 cycle hum. I just completed this radio today and didn't have time to seriously experiment with antennas. But what do you think? Will a proper antenna clean this up?

Incidentally my magnetic wire antenna as of now runs right past the electrical connection to my house. Also, the radio is of my own design. Its input impedance is calculated to be about 4000 ohms in parallel with the input capacitance. I used a high frequency PNP BF979

 

You'll generally have more predictable results with a CLOSED LOOP antenna....they're much more immune to noise and don't require a ground. Look up the many FRAME LOOP antenna projects on the web.

Eric

I intend to experiment. Bertus posted a link to a Frame Loop antenna that I will probably build. And I may put it up on my roof and put an antenna tuner -- an inductor in parallel with a variable capacitor -- in a box on my desk. To get the whole HF band I will use more than one inductor in that box and a switch to go between them. Sound good?

 

I would go with KL7Aj and try a tuned loop first and experiment from there,
the exception being, since you mentioned you're doing the radio yourself,
if it's something pre-superhet such as a TRF with minimal amplification,
then you would want to be going hell for leather with an outdoor wire and earth.

There is a list on the net of wire lengths to avoid if you look for wire antenna
lengths which is the half wave length of any frequency.

It's a super heterodyne receiver. The input rf amp has a gain of 20 at 1 MHz and drives a mixer having a gain of 10.5v/v. This is a module feeding an IF strip with a maximum gain of 1500 but with an attenuator at its front end. I have found I have to turn down the IF strips attenuator substantially to get an intelligible signal into the detector and audio amplifier. So I'm think that first module with its 200+ gain is too much. What do you think?

As far as using a genuine earth ground, I quite agree. I think portable battery radios are losing out on this feature, but they do work.

 

I have been using ferrite rod antenna's on the am band. I read an article sometime ago on that type of antenna. One would think they are horizontally polarized, but from what I recall from the article is that they are really vertically polarized when they are in the horizontal position. It seems the vertical magnetic radio waves are attracted to the ends of the rod.
I have also discovered that if you want to tune for example 600Kc to 1800Kc, you need a capacitor tuning ratio of the square of the frequency ratio. 1800Kc/600Kc = 3:1 so the capacitance ratio is 9:1 I also use a formula LC=25330/fxf.... where LC is inductance in uH,C is in pF and f is in MZ. This formula is a modification to the normal formula fr=1/2∏√LC.

That number 25333 gave me a jolt. Where have I heard that before? Then I remembered. I made a meter to measure inductance by feeding a signal through a 33k resistor into a tank composed of a tantalum capacitor of 1.0 uF and the unknown inductor. This led to the formula L=25333/f*f. The frequency is obtained by sweeping an input signal's frequency, and using an oscilloscope to determine the tank's center frequency.

Incidentally, this 'red box' also determines capacitance using a 100uH inductor and the unknown capacitance is determined in the same way, where C=253/f*f. My red box has served me very well.

 

Well where is it?

 

Well where is it?

Do you mean where is the radio? It's on the desk in my bedroom. I just got it going yesterday. For an antenna I used a length of #30 or so magnetic wire about 30 feet long strung horizontally from the roof of my metal mobile home to the roof of my metal shed.

The radio is noisy but it tunes in the stations we have around here. Since I haven't a standard antenna I don't know if the noise is due to the antenna or the radio. Probably both. I think I gave the radio's front end too much gain -- 200v/v coming out of the mixer. I have to attenuate this at the front end of the IF amplifier in order to make it intelligible.

I'm debating on whether or not to build a good loop antenna next or go back and redesign the front end. I'll try to post the schematic and maybe take some pictures later on today.

 

I meant pictures of the DIY radio
Even the guys at my ham club don't make anything anymore.

 

I meant pictures of the DIY radio
Even the guys at my ham club don't make anything anymore.

Here's a link:

https://www.flickr.com/photos/42069670@N03/13915169296/in/photostream/

 

In the post just above is the schematic of my radio from the front end to the output of the IF amp into the detector. I welcome any criticism.

 

No response? Does this mean my design is perfect?