# Tune a very short 3-5m random wire for 40m (around 7.030 MHz) for listening or occasional QRP CW sending

I would like to use a small trx first for listening CW on 7030 kHz and then for transmitting, locally, among friends.

My conditions are "prohibitive" as for antennas: a block of flats, 4th floor in 8 floor building. Among other blocks of flats. Administration will not allow antennas - i.e.I need to go 'clandestine' or operate from a bench in a park so as not to attract attention.

I live in a capital city where there are plenty of CW addicts, I dream about being able to first listen to them on CW and then respond. No need to go DX; I would be happy to communicate locally.

I was thinking of a random wire matched with a capacitor and a coil but I am not an engineer. What would you recommend? I need only to have it tuned around 7.030 Mhz qrp calling freq and fit my pocket for bench operation or in a window for swl.

• A regenerative receiver can work nicely with a short-length "whip" antenna, if you can cope with the frequency wander due to proximity effect. Simple, small, low-current drain. Great for SWL, but doesn't address your transmitting desire. May 18 '18 at 13:43

Very short antennas have very close to the same directivity as a full size 1/2 wave antenna but their efficiency, and as a result gain, can suffer. The gain of an antenna is given as:

$$Gain = Directivity * Efficiency \tag 1$$

The efficiency is the result of losses in the wire and, in your case, in the matching network that is needed to make the antenna work with your transmitter. So will you want to use the largest gauge wire possible for your situation and make very good solder joints at all connections.

Using EZNEC, I modeled a 6 meter (2 pieces, 3 meters each) center fed dipole in a vertical orientation for this example. The bottom end of the antenna was 7 meters above ground. I did not adjust for building proximity. At 7.030 MHz, the antenna impedance will be 5.4 -j1826 ohms. As a shortened antenna, the 1826 ohms of capacitive reactance is not unexpected. The 5.4 ohms of resistance is low but a manageable number.

A matching network such as this can be used to adjust to impedance to 50 ohms:

simulate this circuit – Schematic created using CircuitLab

I designed this simple L matching network using http://leleivre.com/rf_lcmatch.html. The transmitter side of the matching network is 50 ohms so a convenient length of RG8X coax could be connected to this point and run to the transmitter. The coil should be made of fairly large diameter wire or even 1/4 inch soft copper tubing from the hardware store.

For the inductor shown, a construction example would be 0.25 inch soft tubing wound 10 inches in diameter, 7 inches long, and with 14 turns. The circuit can be adjusted by slightly expanding or compressing the coil. Keep in mind that this is a very big coil that must be placed at the center of dipole to optimize the efficiency but you may be able to cleverly use it to support the antenna. Many other coil geometries are possible. You can calculate alternate coil details using a site such as http://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml.

This may not be the exact antenna you envisioned but it gives you an idea how to go about designing a compromise antenna. At your building height, you should give thought to using a vertical or horizontal, shortened dipole antenna. It may serve you well if you pay attention to construction details that can affect efficiency and thus gain.

Clandestine can be very effective. A thin wire hanging out of your window is hard to see, especially if you only have it outside while operating. Commercially built tuners aren't expensive either.

Another option to consider is MCW (modulated CW) where you send the Morse tones over audio via VHF/UHF FM. If you have enough CW fans there you may be able to get enough interest. Or for that matter, VHF or UHF CW. It's far less common, but not unheard of.

For listening, you can use any length of wire you like (longer than this will be better at that frequency, but even a short wire will pick up strong transmissions).

As for transmitting, you would be far better off using a higher frequency, given your antenna limitations. Depending on the range you require, the 10m or 6m band will work with most HF radios, and if you have a capable transmitter, line-of-sight CW on 2m or 70cm or even shorter bands might be worth considering, too. You don't say what country you're in but this would all be dependent on having the appropriate license and the appropriate band being legal in your country for amateur radio.

One of my favorite things to do is QRP field operation with compromised antennas. The easiest is just a piece of wire with one end thrown over a tree branch and a counterpoise wire laying on the ground. I use a sling shot to launch 20 lb fishing line up and over a branch (up to about 15 to 20 feet high). My antenna is a single wire about 40 to 65 feet long. And, yes, an antenna tuner is required. The antenna tuner built in to my Elecraft KX2 QRP rig handles just about any wires I have put up. The counterpoise wire laying on the ground is absolutely required for good performance. Adjusting its position on the ground relative to the wire on up in the air is helpful to getting a good match.

My best performing antenna for field/portable work though is my Buddipole antenna. Although I work primarily CW on 20 meters, I have had many contacts on 40 meters too. Yes, you have to spend money but I consider that money well spent.

As others pointed out, the shorter the antenna, the lower its efficiency. Trying to use short wire in combination with a QRP transmitter is very likely to be a very frustrating experience (I know this from my own experience).

If I simplify, it is the current in the antenna that causes RF transmission. Therefore, if your antenna is a short wire, it will always be inefficient. This is because the current at the far end of the wire is always zero (obviously, it has nowhere else to go...) and it grows gradually towards the feedpoint and it reaches its maximum at quarter wavelength from the end. Short wire (less than 1/4 wl) is always too short to develop sufficiently high current at the feed point. (The coil shown in the first answer will only match the antenna impedance to the output impedance of your transmitter, but it does not create more current in the antenna, it is physically impossible).

That said, using a small magnetic loop antenna tuned to your transmitting frequency, made of high-quality materials (copper tube at least 1/8 inch in diameter, vacuum or air capacitors able to withstand high RF current) may be a better option, because its efficiency can be much higher (compared to very short wire). The reason is that in small MLA (which is in fact just a physically big resonant circuit) you can achieve high RF current thanks to high Q of such antenna, whereas with a short wire you have absolutely no chance to push more current into the antenna than its physical length (relative to wavelength) allows.

However - the small MLA also has a problem. It is highly sensitive to any electrically conductive objects in its vicinity. They could detune the antenna or even absorb some of the RF energy by electromagnetic induction caused by the high RF current in the antenna, or by capacitive coupling. MLA is actually very similar to the induction ovens used to heat vacuum tubes before sealing.

I am now thinking about g4fon magloop. http://www.g4fon.net/MagLoopTwo.htm Contemplate using RG58 coax shield as radiator. 0.5mm dia.

Antenna efficiency: 30% (-5.3 dB below 100%)
Antenna bandwidth: 34.2 kHz
Tuning Capacitance: 126 pF

Capacitor voltage: 333 volts RMS
Resonant circulating current: 1.85 A
Loss Resistance: 0.308 ohms
Inductance: 4.08 microhenrys
Inductive Reactance: 180 ohms
Quality Factor (Q): 205
Distributed capacity: 19 pF

Antenna "circumference": 7 meters
Loop antenna    Side length: 0.875 meters
Antenna diameter: 2.1 meters