An 88-108MHz LPA for Commercial FM

With the common availability of DAB, the old analogue FM frequencies seemed to be getting pushed to one side these days. However, if you turn your dial anywhere between 88 and 108MHz, the chances are you hit a deluge of commercial stations. Many will be local to your area running modest power levels, but some pump out many tens of kilowatts and some will be QRP serving very limited local communities.

Some years back, I was approached by an enthusiastic Band II FM DX’er who asked if I could build a directional antenna for Band II that would cover the whole band and with some good gain and pattern. There’s quite a few of these die-hard enthusiasts still around and many have some premium high-end receivers. However, the general feedback that I was getting was there was a very distinct lack of a good gain directional antennas for this band that are built to withstand some serious punishment from the weather.

Here’s a plot from 95MHz. Great gain and pattern. Notice the very flat SWR across the whole 20MHz

I’d only constructed a handful of LPA’s mainly using LPCAD software by W8IO. Generally, the builds created with LPCAD seemed to match NEC2 calculations fairly well. Not having EZNEC4 didn’t seem to be too much of an issue so I got to work designing an handful of differing LPA’s to cover from 88 to 108MHz. A fairly straightforward 5 element design looked ok but lacked gain and with the limited number of elements seems to struggle to cover the whole band. Upping this to 7 elements mainly solved the issue and created a nice sturdy antenna with a boom length of around 3m and nice gain/patterns.

Initial set and testing on a couple of workbenches. Phasing lines can be seen between the elements.

The LPA was designed and based around a 75 ohms coaxial feeder which would match the 75 ohms input of the user’s commercial receiver. Gain was between 9.04 and 9.70dbi with the front to back at around 20db at its lowest point which was around the lowest frequency [88MHz], rising to over 35db at some frequencies.

Each phasing line is twisted 180 degrees and should be crossed over
Most models show an improved front-to-back ratio on the lower frequencies when a shorting stub is used
The feed point is always at the front of the array. Hook up the feeder to the split-dipole centre of the shortest element

Construction Data:
Boom: 1 x 3.1m of 1.25 x 1.25 inch x 10 gauge box section aluminium [this is not a split dual-boom design]
Element Mounting Plates: 7 x 1/4 inch aluminium plates secured by 2 square section U-bolts
Elements: Inner sections, 16mm [or 5/8ths] x 16 gauge aluminium tubes
Elements: Outer sections, 12mm x 2mm wall aluminium tubes
Elements: All ‘Split-Dipole’ style all isolated from the boom
Element Clamps: Stauff Standard Duty Group 2 Polyamide [216PA]
Inter Element Phasing Line: 75 ohm Twin Lead, each element is connected opposite so a 180 degree phase shift exists. We found that 14 AWG speaker cable came very close to 75 ohms
Feed-point: At the front of the array [smallest element] split-dipole feed
Longest Element: Small 5cms short-circuit link wire between the dipole pins

As expected, the SWR curves when using a 50 ohm analyser and feeder were slightly high. Our model was based on a pure ’75 ohm system’ and feeder. Judging by the analyser readings below, we weren’t far out at all. Subsequent checking by the buyer confirmed a nearly flat response from 88-104 MHz and rising to 1.3:1 at worst. You could of course feed it with a 50 ohm coax system via a 1.5 to 1 balun at the feed point – so both options are very useable.

Download the PDF Plots below:

Download the EZNEC File below:
EZNEC file for reference.

Here’s some feedback from a buyer:
Good evening Steve,
I hope you are well and enjoying your retirement despite the Covid restrictions.

Back in 2015 I purchased a 7el log periodic from you for FM Band 2, and in the last month I finally got round to building it – the pace of life is slow in Sutherland, and projects take a while to come to fruition!  I am writing to congratulate you on producing such a well-engineered item.  So many commercial designs these days are flimsy and produced to a price, not a quality, so it’s refreshing to find I’ve purchased such a high-quality item.  The bag of spares was the icing on the cake.

I’ve no doubt that this antenna will stand up to the coastal gales we experience here on the north-west coast of Scotland and bring in some good Sporadic-E DX over the summer (we don’t get much tropo up here).  It’s working well with the Elad FDM-S3 SDR.  Photos attached for your enjoyment.  Thanks for producing such a good antenna.

73, Martin