How to make small (< 1m) HF antenna?

[Dobo]

I am looking to do some experiments with short range (< 100m) HF signals using small antennas, but I know very little about antennas and how to make them efficient.

I want to transmit a 5 MHz carrier and pick it up at ranges from 10m - 30m. I want to use small antennas to do this. When I say small I mean something portable, preferably < 1m on the largest dimension. Furthermore I'd like to be able to test both monopole/dipole and a loop antenna.

Considering that a 5 MHz signal has a wavelength of ~60 m any antenna with those dimension are bound to be pretty poor in itself and will need some matching.

My idea for making an okish monopole is to simply simulate a monopole of a given length, say 60 cm, find the impedance and then make a matching circuit that can match from a 50 Ω coax to a whatever Z the antenna is.

For making the loop I figured using the formula for an LC circuit:
Frequency = 1 / (2 * pi * sqrt(L * C))

Wind a loop, calculate the inductance and use the above formula to find the approximate C and then tune the C until the resonance hits the right frequency.

For both I'll use ferrite beads clipped around the coax as chokes/baluns.

I'm using lab equipment for this part, so a signal generator generates the signal and I check for a signal on a 200 MHz oscilloscope.

Is this approach feasible? Do I need something other than the matching circuit for the monopole and does the loop require a matching circuit? Is the ferrite beads enough, or do I need something more to stop signals running down the coax shield?

Any help and pointers are welcome.

**

Looking at this thread:

http://www.eng-tips.com/viewthread.cfm?qid=346804

it seems only matching at the transmitter side is required?

 

[VEBill]

30m range is not very challenging. If your receiver is at all sensitive, there should be no problem picking up milliwatt class signals, even with inefficient antennas, over that range. Note that I used the word Receiver, vastly more selective and sensitive than an oscilloscope.

Keep in mind that 5MHz is the HF band. 5.000... MHz is assigned to the NIST standard station WWV. On the one hand, you should not interfere with licensed users. On the other, your receiver may be picking up a strong signal from the assigned user and interfere with your experiment. If your signals are ultralow power, you might get away with playing around on random frequencies, but you should be quite careful to avoid interference in either direction. In general, you should have a receiver to avoid transmittng blind.

Demand for small antennas is a recurring theme in both the hobbiest world (ham radio) and military users. In general, feed point impedances are very low, matching is difficult, and resistive losses become significant.

 

[Dobo]

Thanks for the reply.

I know it's not a free frequency, but this is just for a lab test with low power signals. I'll look out for interference though.

Good point on the receiver vs oscilloscope, will see if I can get my hands on a receiver.

With regard to the matching. Is it needed and if so do you have any good references I could study?

 

[VEBill]

Depending on the purpose of your experiment, I'd try it first without bothering to spend too much time on matching. Only if the signal is not detectable at 30m would one need to bother investigating the match.

You can use the inverse-square law to compare your best range to how many dB you have to spare (or are missing).

There's tons of "short HF antenna" information on the 'net. Here's a good one to get you started:

http://www.n3ox.net/projects/n3oxflex/

 

[zappedagain]

A decent scope (Tek 3052) with FFT capability can give you a -80 dBV noise floor (approx -76 dBm). That might be good enough for a proof of concept. AM? FM? PM? Modulation rates?

Z

 

[Dobo]

Actually just a pure carrier, no modulation.

I'm looking to measure near-field properties of signals and how they differ between antennas. More specifically I want to see if I can measure the difference in phase between the electrical and magnetic field that exists in the nearfield. That's why I need both a monopole (electric) and a loop (magnetic) antenna.

So I figure an easy way would be to get both signals on the same scope.

 

[Higgler]

The most difficult antenna to test accurately is the low frequency antenna. Expect your environment (and the RF inadvertent bounces off that environment) to swamp out results.
i.e. take some measurements, move test locations, totally different measurements. Potential waste of time.

 

[Sawbux]

Feed into a dummy load and extend a wire from the center conductor of the coax the desired length.

Almost anything can be an antenna. Many a ham has had his transmitter attache to a light bulb as a dummy load and made contacts with it.