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PC board log periodic antennas

-December 15, 2012

Always on the lookout for low-cost and small broadband antennas that I can pack into my EMC troubleshooting kit; and having seen a few displayed at the EMC symposia, I thought I'd look into PC board designs. It wasn't long before one antenna designer rose to the surface in my research - Kent Britain (WA5VJB). Now, I'd known of Kent from past amateur radio microwave symposia and other VHF+ radio activities, but I'd forgotten he also specialized - and sells - a number of off-the-shelf and custom PC board antenna designs. I decided to order three types of log periodic (LP) antennas designed for the frequency range, 400 MHz to 11 GHz. Together, they cost me all of $53. He also sells an interesting Vivaldi ultra-wideband (UWB) antenna covering 5 to 18 GHz for just $6 (pictured at the end of the article in figure 7 below).


Figure 1 - The three models of log-periodic antennas.

Here are the specs and pricing for the various models:

Freq (MHz)

Approx. Gain (dBi)

Price (USD)

400 - 1000

6

28

850 - 6500

6

18

2 - 11 GHz

6

7


After ordering them through the Kent Electronics web site, they soon arrived and I installed suitable PC-mount SMA connectors at the tips - mounting each on the "ground" side of the array and running the coax cable straight down the center.

As a nice touch for us EMC engineers, Kent has measured the antenna factor (AF) for each antenna.

400 - 1000 MHz LP

Freq (MHz)

AF

400

18.8

450

17.7

500

18.2

600

19.8

700

21.2

800

22.3

900

23.3

1000

24.2

1050

24.7

850 - 6500 MHz LP

Freq (MHz)

AF

900

24.0

1000

24.2

1500

27.5

2000

30.1

2400

32.0

3000

33.0

6000

40.0

2 - 11 GHz LP

Freq (GHz)

AF

2

30

3

33

6

40

10

45

Construction - I wanted to be able to disassemble and store the antennas and a small table-top tripod in my troubleshooting kit, so I fashioned a mounting scheme based on standard 3/4-inch PVC pipe. After a bit of experimentation at the local hardware store, figure 2 shows the components I decided to use. By drilling and tapping an end cap with a 1/4-20 thread, it will screw onto a standard photographic tripod. Then, I screwed the 90-degree adapter to the cap, hand-tightening the two together.


Figure 2 - Here are the various components used for the antenna mount. The end cap was drilled and tapped with a standard 1/4-20 thread size, which is a standard photographic tripod mount.

Using a hand saw, I carefully cut a slot partway into the horizontal pipe and pressed (or glued it, if required) to the rear of the LP as shown. The other end of the horizontal pipe was simply pressed into the 90-degree adapter, so that I could easily rotate the antenna for horizontal or vertical polarization.


Figure 3 - Photo showing the completed antenna mounted to the table top tripod.

The tripod used was a table-top model "T-Pod" from www.trek-tech.com ($49.95). It collapses and is stored in a small pouch - easily packed into the troubleshooting kit. The sections connect together with strong neodymium magnets, so it's structurally strong and holds the antenna without any trouble. Other small-sized tripods may be used, as well.


Figure 4 - The T-Pod table top tripod collapsed and ready to stow inside the supplied case.

I measured the VSWR of the antenna from 200 to 1,500 MHz and it looked very good over the advertised range of 400 to 1000 MHz - mostly staying better than 2:1 across the band. There were several larger peaks near 1:1. Interestingly, there was also good resonance from 1,200 to 1,500 MHz - better than 2:1, or so.


Figure 5 - The measured VSWR response of the 400 to 1000 MHz LP antenna. The marker is located about 1,000 MHz and indicates a VSWR of 1.66:1 - not too bad! The four higher peaks are all near 1:1 VSWR.

Kent has several good antenna design references on his web site. Among them, "Designing PCB Log Periodic Antennas".

I wish the 400 to 1000 MHz LP would go lower in frequency, but then it wouldn't fit so nicely (barely) in the Pelican 1510 roller case I use for my troubleshooting kit. Also, in discussing the issue with Kent, he said the size was at least partly limited due to the maximum panel size used by his PC board fabricator. So, I'll continue to use the "rabbit ears" TV antenna for troubleshooting at the lower frequencies. One nice feature of these antennas is that Kent has added a small PC board inductor at the rear of the antenna that effectively grounds the two arrays together. From Kent's "Designing PCB Log Periodic Antennas:

"Designing a Log Periodic to be grounded at the back of the boom helps solve many problems. The entire antenna is at DC ground and many low frequency response problems are eliminated. In high EMI areas, ungrounded LP's can look like a fat 1/4 wave ground plane to VHF signals and let in intermodulation sources. Removing the via will lower the lowest usable frequency slightly, but introduce many new resonance's into the antenna."


This "DC short" should also reduce the chances of ESD getting into the sensitive front-end of your spectrum analyzer.

To use these antennas for troubleshooting, simply set them up on one end of a table or workbench, with the product under test at the other end. Try to get at least 1m distance between the two. Now, connect the antenna up to a spectrum analyzer, via a broadband preamp, if necessary. While you do your troubleshooting on the product, you can watch the immediate results on the analyzer!


Figure 6 - Kent Britain shown here "adjusting" a product under test at an EMC test range in China. Photo, courtesy Kent Britain.


Figure 7 - Another cool antenna design is the Vivaldi ultra-wideband (UWB) PC board antenna, which is resonant from 5 to 18 GHz with typical gain of 8 to 10 dBi. The cost is just $6.

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