SPIRAL ANT

[HL1SAK]

 

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[SM�VPO 80 m spiral loop]    [SM�VPO-style 40 m spiral loop]   [SM�VPO-style 20 m spiral loop]

One of my 2008 antenna projects was a square spiral loop antenna for the 80 meter band. It is designed by Harry, SM0VPO. So far I have not been active on 80 meters, due to antenna limitations and its perceived suitability for DX. So I thought I'd give this one a try.

"80 meter frame antenna" by Harry Lythall, SM0VPO.

This antenna is very compact: ≈ 120x120 cm (4x4 ft). It is basically a transformer loop antenna: it consists of a tuned loop and a small feed loop. The tuned loop is spiral-wound and square, has about � λ total wire length, and is terminated with a variable air capacitor. The small coupling-loop (feed loop) is triangular with a wire length of about 0.03 λ (≈2m75, ≈ 9 ft). The latter is center-fed and connects directly to 50 Ω coax.

Source: "80 meter frame antenna"

A "high voltage" type tuning capacitor is required, as very high voltages will be generated across the capacitor! The original design calls for a 25 pF variable capacitor. I used what I had in my junk box: a decent  5-15 pF trimmer capacitor. This was fine for initial measurements with my antenna analyzer and low power (10 W) tests. It has 1 mm distance between the plates, so it should be OK for about 1 kV. I later bought several 5-75 pF high voltage air variable capacitors (Voltronic model NTM70 6E). They are very compact: 7� cm long (3"), 1.6 mm diameter (5/8"), and it takes 120 turns of the shaft for end-to-end travel. I bought them on-line at Max-Gain Systems. I got two with the standard rating of 3 kV (6kV peak), and two tested to 15 kV. Mid-2009 pricing was a reasonable $29 and $39 each, respectively (plus $13 for shipping them to my QTH in France). Now I have to design and build a remote-control motor drive for them...

As the antenna analyzer plot at the bottom of this page shows, the antenna has an SWR=2 bandwidth of approximately 25 kHz at 3578 kHz.

  
Photo 1: 5-15 pF air variable trimmer capacitors (left), high-voltage 5-75 pF air variable capacitor (right)

Another spiral loop is described here:

"Mark 2 QTC 80/40 loop antenna", by Richard Marris (G2BZQ), Elektor Electronics, July 1992, pp. 88-90

CONSTRUCTION

Components:

	25 meters of 0.75 mm2 insulated wire ("zip wire", AWG 20 or 21, not critical); as suggested by Harry, I split 13 meters of  2x0.75 mm2 household hook-up wire (zip wire), and made a 26 m wire out of it.
	25 pF high-voltage variable capacitor (at least several mm separation between the plates).
	2-hole household terminal strip.
	BNC or SO-239 coax connector.
	2 sections of (at least) 180 cm (6 ft) PVC tubing, at least 16 mm (5/8") diameter, from your local Do It Yourself or building supply store.
	PVC T-piece, for 5 cm (2") diameter PVC pipe.
	6 meters of thin mason's string (e.g., 1 mm diameter non-stretch/pre-stretched multi-strand dacron or similar; nylon stretches way to much).
	A short piece (5 cm) of shrink tube for the antenna wire.

Photo 2: the main components, other than the "arms" of the square loop

Construction is simple. Drill 3 mm holes through the 16 mm pipe, at the distances from the center as marked in the diagram below. At the very tips of the pipe, drill a small hole through the pipe, in a direction perpendicular to the other holes. This will be use to connect the tips with a mason's string, to help the pipe "cross" from changing its shape when we install the antenna wire (the tubes are flexible).

Make a hub for the crossing arms of the antenna structure. I used a PVC T-piece for this, intended for PVC pipe with a 5 cm (2") diameter. All my home-brew antennas are low-weight, and my "mast" is a section of 5 cm pipe. Drill the holes such that it is a tight fit for the 16 mm � PVC pipe! I pre-drill with a 15 mm drill (start out with smaller diameter, then increase to 15 mm), and finish with a conical grinding bit. See photo 3 below.

Then slide the PVC tubes through the hub and center them. Turn the pipes such that the drill holes in each pipe face the other pipe. Next, use the mason's string to connect the tips of the pipe - the "arms" of the cross must remain straight, and form a cross, such that the loop is square. See photos 4a and 4b. This helps a lot when stringing the antenna wire through the arms!

Tie a knot in the antenna wire, about 25 cm (≈10") from the end. Thread the other end through the outer-most 3 mm hole in one of the arms. From there, thread it through the outer-most hole in the adjacent arm. Put some tension on it (not a lot!). I clamp a hemostat on the wire, right at the "exit" hole. If you don't have one of these medical instruments, you can improvise with small pliers and a heavy rubber band on the handles. Now thread the wire the outer-most hole in the adjacent arm, hold the tension and move the hemostat or pliers to this "exit" hole. Repeat until you have made a 5-turn loop. When you put some tension on the wire, the arms of the cross will bow; this is OK and actually helps maintain tension.

Leave about 25 cm wire sticking out of the final hole, and cut off the rest of the wire. Slide the piece of shrink tube over the wire, all the way up against the final "exit" hole (see bottom of photo 5), and - while maintaining tension on the wire - shrink the tube with a hairdryer or soldering iron. 

Attach your variable capacitor somewhere on the pipe section that has the start and end of the loop wire. I used a couple of tie-wraps for this. Trim the wire ends to length and connect them to the trimmer. I also fixed the wire ends to the pipe with some tie-wraps.

Now install the feed loop. This is a small triangular loop. A right-angled isosceles triangle, to be precise (two equal-length sides and one 90� angle). It has two sides of 80 cm (79 in my case), and a hypotenuse of about 114 cm. So we'll need about 3 meters (10 ft) of antenna wire. We need to make two tie-offs out of mason's string, such that the 45� angles of the feed loop are at a distance of 4 cm from the arms. The tie-offs are tied to the innermost 3 mm hole in the arm (i.e., the one closest to the hub). See photo 6. String the antenna wire through the innermost 3 mm hole of the arm that has the start and end of the spiral loop. Position it such equal lengths of wire stick out of the pipe. Thread each wire end through a 5 cm long tie-off, put some tension on them, and lead them to the hub. Mount the terminal block on that same pipe, close to the hub. See photos 7a and 7b. Cut the wire ends to length, strip off � cm of insulation, solder the wire, and mount them in the terminal block. Mount a coax connector just above the terminal block, and wire it to the same terminal block.

My antenna weighs 800 grams.

When the construction is complete and checked out, I will house the capacitor in a small waterproof box, and weatherize all electrical connections with a good coat of DipIt/PlastiDip.

That's all! Now you can erect the antenna (see photo 8).

Photo 3: the PVC T-piece, with holes drilled for the 16 mm diam. arms

Photo 4a: the square loop - spreaders fixed in place with mason's string

Photo 4b: the square loop - spreaders fixed in place with mason's string

Photo 5: a trimmer capacitor closes the spiral loop

Photo 6: attachment of the feed loop

Photo 7a: details of the "hub" of the antenna

Photo 7b: details of the "hub" of the antenna

Photo 8: the antenna erected on my terrace
(crossing in the background is my Cobra dipole antenna)

MEASUREMENTS

For starters, I hooked up my miniVNA antenna analyzer directly to the coax connector at the antenna. I measured the SWR and R of the antenna, both for the trimmer at its minimum and maximum value. I could easily tune it between 3.64 and 4.3 MHz. See plots below. These plots show a very nice resonance dip of the SWR, and an impedance at resonance very close to 50 Ω - no tuner required. What more could you ask (other than radiation efficiency)? The SWR varied from ≈ 1:1.1 to 1:1.4 for these two extreme trimmer settings. SWR=2 bandwidth appears to be several 100 kHz.

As I am interested in the low end of the 80 meter band (3550 - 3650 kHz), I will need to get a larger capacitor...

The second set of plots below show a full HF sweep from 1.5 to 30 MHz. With the antenna outdoors, there are additional dips at multiples of the base resonance frequency. As to be expected, they don't coincide with 50 Ω impedance frequencies.

When I get a high(er) voltage trimmer cap, I'll run some operational tests and report the results here.

I actually decided to construct a 40 meter version of this antenna.

Photo 9: antenna analyzer hooked up directly at the antenna

 
Resonance with low SWR and 50 ohms impedance at 3.65 MHz (trimmer at max) and 4.2 MHz (trimmer at min)
(ignore the bandwidth/Q indications, they are not valid as I did not correctly select the marker frequencies)

 
Sweep from 1.5 to 30 MHz - indoors (left, directly at the antenna) and outdoors (right, via 15 mtrs of coax)
(ignore the bandwidth/Q indications, they are not valid as I did not correctly select the marker frequencies)

Resonance with low SWR and 50 ohms impedance at 3.578 MHz (with 15 kV variable air capacitor of 5-75 pF)
(ignore the bandwidth/Q indications, they are not valid as I did not correctly select the marker frequencies. Here SWR=2 bandwidth is actually abt. 25 kHz)

[construction]   [measurements]

Last update: 16-Aug-2009. I have built the 80 meters spiral loop antenna designed by Harry, SM�VPO. My construction and analyzer measurements are described here. It is basically a mono-band antenna. As I am interested in adding a 40 meter antenna to my antenna collection, I decided to adapt the 80 meters spiral loop for operation on 40 meters. These antennas are easy to make and are inexpensive, so I gave it a try.

I made a quick-and-dirty Excel spreadsheet to play around with the dimensions. In the 80 meters version, the total wire length of the triangular feed-loop is about 0.03 λ. I decided to apply the same ratio for the 40 meter version. I also retained the distance of 5 cm between the innermost spiral winding and the feed-loop, as well as between the spiral loop windings themselves. The total wire length of the spiral loop is about 1/4 λ. Start and end of the spiral must be on the same spreader arm. The spreadsheet showed that this can only be done with a 4-turn spiral, instead of the 5-turn spiral of the 80 meters version.

The resulting antenna is not 50% but 33% smaller than the 80 meters version: it measures about 80 cm square, compared to 120 for the 80 meters version. Very compact indeed! It also connects directly to 50 Ω coax, and uses a 25 pF trimmer capacitor for tuning. With 450 grams, it weighs a tad over half of my 80 meters version.

As with the 80 meters version, I have not been able use the antenna with my transceiver (other than for receiving): I don't yet have the required high-voltage (several kV) trimmer capacitor. A "high voltage" type is required as very high voltages will be generated across the capacitor. I used the same "regular" 5-15 pF trimmer capacitor that I used for my 80 meter version of this antenna.

CONSTRUCTION

Components:

	14 meters of 0.75 mm2 insulated wire ("zip wire", AWG 20 or 21, not critical); as suggested by Harry, I split 7 meters of  2x0.75 mm2 household hook-up wire (zip wire), and made a 14 m wire out of it.
	15 pF high-voltage variable capacitor (at least 2 mm separation between the plates).
	2-hole household terminal strip.
	BNC or SO-239 coax connector.
	2 sections of (at least) 120 cm (6 ft) PVC tubing, at least 16 mm (5/8") diameter, from your local Do It Yourself or building supply store.
	PVC T-piece, for 4 cm diameter PVC pipe ( I used 5 cm diam. for my 80 meters version but decided to go down one standard size. Works just fine. A standard reduction piece adapts this to my 5 cm diam. PVC mast).
	4 meters of thin mason's string (e.g., 1 mm diameter non-stretch/pre-stretched multi-strand dacron or similar; nylon stretches way to much).
	A short piece (5 cm) of shrink tube for the antenna wire.

The main components, other than the "arms" of the square loop

Construction is simple. It is the same as what I described here for the 80 meters version (with dimensions adapted per the diagram below).

The 80 and 40 meter versions side by side

MEASUREMENTS

For starters, I hooked up my miniVNA antenna analyzer directly to the coax connector at the antenna. I measured the SWR and R of the antenna, both for the trimmer at its minimum and maximum value. I could easily tune it between 6.8 and 8.5 MHz. See plots below. Again, a very nice resonance dip of the SWR, and an impedance at resonance very close to 50 Ω - no tuner required.  The SWR was ≈ 1:1.3.

The second set of plots below show a full HF sweep from 3 to 30 MHz. There is an additional resonance dip around 23.5 MHz (3rd harmonic of the base resonance?), but I did not check where this one goes when changing the setting of the trimmer capacitor).

I did a very quick "sound check" with my receiver. Received signals were at least as strong as with my short Cobra dipole. I did not turn the antenna to get a feel for its directionality. The trimmer cap that I am using now has 1 mm distance between the plates, so it should be OK for about 1 kV. I have tuned the antenna with 70 watt and there were no signs of arcing. I'm not sure how high I can go without arcing and I don't want to destroy the trimmer. Supposedly the voltage Vc across the capacitor is Q x Vo, where Q is the "Quality" of the LC-circuit (resonance frequency divided by the bandwidth), and Vo the oscillator (transmitter) voltage.

The analyzer plots show a Q of 202. I assume that the output transistors of my transceiver are powered by 12 Vdc (or 13.8). That would result in about 2800 Vdc at the antenna... More on this (including actual arcing) in the description of my 20 meter version of this antenna.

(ignore the bandwidth/Q indications, they are not valid as I did not correctly select the marker frequencies: SWR=2 bandwidth is actually abt. 35 kHz)

   
Resonance with low SWR and 50 ohms impedance at 6.8 MHz (trimmer at max) and 8.5 MHz (trimmer at min)
(ignore the bandwidth/Q indications, they are not valid as I did not correctly select the marker frequencies)

 
Sweep from 13 to 30 MHz - outdoors via 15 mtrs of coax

Update: 9 June 2010. Bill, W7ZT, built the 20 m spiral using my design, and is happy with it:

"Frank: Thanks for info on the Spiral Antenna. Built the 20 meter version today and works wonderfully !!

I found a 9-58 in the junk box that worked on the top end of 20 meters, but was very narrow banded. I'm looking for an air cap to allow the full band. Did you build a motor drive for the air capacitor ?? If so, can you share details.

See attached for my newest fun antenna !! Took the picture before I "cleaned" it up. The BNC to Binding post worked well for input.

I get great pleasure in building QRP antennas. Been a Ham for 52 years and having more fun now than ever.

Thanks again Frank..

Bill W7ZT

Update: 17 August 2009. After having built the 80 meters spiral loop antenna designed by Harry, SM�VPO, and successfully scaled it down to my 40 meters version. For fun, I decided to try the "limbo approach", in other words: "how low can you go?"

I updated the Excel spreadsheet that I made to arrive at the 40 meters version, with the possible solutions for a 20 meter version. Again, retaining the circumference of the triangular feed-loop at about 0.03 λ, I had to reduce the number of loops by one to three (from 5 for the 80 meters, and 4 for the 40 meters version).  Again, going to half the wavelength reduced the size of the antenna by 1/3 to 60 cm square (down from 120 cm for the 80 meters, and 80 cm for the 40 meters version). Weight has gone down to 340 grams (from 800 grams for the 80 meters, and 450 grams (1 lb) for the 40 meters version).

CONSTRUCTION

Components:

	7 meters of 0.75 mm2 insulated wire ("zip wire", AWG 20 or 21, not critical); as suggested by Harry, I split 7 meters of  2x0.75 mm2 household hook-up wire (zip wire), and made a 14 m wire out of it.
	15 pF high-voltage variable capacitor (at least 2 mm separation between the plates).
	2-hole household terminal strip.
	BNC or SO-239 coax connector.
	2 sections of (at least) 80 cm (2 ft 6") PVC tubing, at least 16 mm (5/8") diameter, from your local Do It Yourself or building supply store.
	PVC T-piece, for 4 cm diameter PVC pipe (I used 5 cm diam. for my 80 meters version but decided to go down one standard size. Works just fine. A standard reduction piece adapts this to my 5 cm diam. PVC mast).
	A short piece (5 cm) of shrink tube for the antenna wire.

Construction is simple. It is the same as what I described here for the 80 meters version (with dimensions adapted per the diagram below). As the PVC tubes are only 80 cm long, they are stiff enough to not need a guy wire strung between the tips of the tubes.

 

My 80, 40, and 20 meter spirals side by side

The 20 m spiral built by Bill, W7ZT

MEASUREMENTS

Again,  I hooked up my miniVNA antenna analyzer directly to the coax connector at the antenna. I measured the SWR and R of the antenna, both for the trimmer at its minimum and maximum value. I could easily tune it between 12.1 and 16 MHz. See plots below. Again, a very nice resonance dip of the SWR, and an impedance at resonance very close to 50 Ω - no tuner required.  The SWR was ≈ 1:1.3 - 1.6. Tuned to 14230 kHz with my 15 kV variable capacitor, the analyzer claims a Q of 127.

The transistors in the output amplifier of my transceiver are powered by 12 Vdc (or 13.8). My understanding was that this would result in a max of 13.8 x Q = 13.8 x 127 ≈ 1750 Vdc at the antenna - definitely exceeding the rating of my original trimmer caps. I read somewhere that for Vac, you need to double that rating... Tuning at low power (≈ 10 W), everything looked fine. As soon as I changed to high power (about 70 W in my case), the SWR meter pegged! While continuing to transmit a test tone, I had to completely de-tune the two variable caps of my tuner and then re-tune back to the previous settings. Suspecting some "ionization" effects at the trimmer capacitor at the antenna, I asked my girlfriend to have a look at the antenna while I keyed the transmitter (it was night time): "Yes, the little light bulb is glowing!". Say no more... The small trimmer cap was enveloped in a light blue luminous ionization discharge aura. No visible arcing damage, though it only takes tiny damage spots on the capacitor plates to significantly reduce the voltage rating! Time to search for a variable capacitor with a (much) higher voltage rating. Several trimmer caps actually, as I am using the same type in my other spiral loops.

 
Resonance with low SWR and 50 ohms impedance at 12.1 MHz (trimmer at max) and 15.8 MHz (trimmer at min)
(ignore the bandwidth/Q indications, they are not valid as I did not correctly select the marker frequencies)

Tuned to 14230 kHz (SSTV frequency): SWR 1.12
(ignore the bandwidth/Q indications, they are not valid as I did not correctly select the marker frequencies