Simple FM antennas: Introducing the Half Wave Folded Dipole

Cost:  From $ 45-75

Length: Approximately 137 cm (54″)

The Half Wave Folded Dipole comprises two elements of typically Aluminum Alloy construction. The Folded Dipole forms the active (or driven) element in a Yagi-Udi array.

Folded dipole for DAB+ bands © 2013 Digitek

Folded dipole for DAB+ bands © 2013 Digitek

How is it used?

This directional antenna is typically mounted ‘broadside’ to the desired transmitter. Experimenters may choose to add additional dipole elements or incorporate reflectors and directors.

How do I buy one?

If looking to purchase one, there are several options:

  • Remove this element from an inexpensive FM yagi. Before purchasing a yagi to ‘harvest’, please check with the retailer or manufacturer to ensure that the element can be removed; on some retail FM antennas (including the Matchmaster & Hills three element FM yagis) the folded dipole can NOT be physically detached.
  • Rescue Electronics Surplus of Connecticut custom manufactures a ceiling-mounted folded dipole. This antenna is ideal for bedrooms; cost is from $99 with worldwide shipping possible.
  • D-lenp of China manufacturers a two element FM yagi with detachable folded dipole. Cost is $25 plus shipping. Five element varieties are also available.

How do I make one?

At the end of this article there are links to Do It Yourself (DIY) antennas that enthusiasts or radio amateurs have constructed with copper tubing for FM broadcast, digital television & even 850 MHz wireless internet reception!

A folded dipole for FM may be constructed with 19 mm (0.75″) square tube aluminium extrusions or even 6, 8 or 10 mm round tube extrusions.  The choice of round tubing is recommended. Although one metre (3.3 ft) long extrusions (at several of these diameters) are widely advertised for a few dollars each from a major hardware retailer, it is usually necessary to make a custom cut of the appropriate length.

To determine the required lengths for each of the three sections of the folded dipole, please consult an on-line dipole antenna calculator or a step-by-step guide to FM yagi construction. During October*, a step-by-step DIY project will be published on this blog as part of this series on simple FM antenna building. Readers can easily and inexpensively make their own folded dipole for FM reception from copper tubing.

A potentially easier solution than the use of aluminium or copper tubing is to construct a folded dipole (or a standard single dipole) from copper or aluminium foil tape attached to a round tube PVC pipe. And better still; the total cost is far less than a round of drinks with a few lads at The Local.

Is it suitable for field trips?

Absolutely! (Most antennas are, of course but the lighter and smaller to carry the better!)

Half Wave Folded Dipole & suppression choke in sedan © 2013 FM DXing at WordPress

Half Wave Folded Dipole & suppression choke in sedan © 2013 FM DXing

How does it perform in the field?

Whenever this writer ventures into the mountains, this antenna is employed since it outperforms the factory monopole in the car. Whilst this antenna design has long been studied using computer modelling by researchers, empirical observations in the field are (hopefully) a bit more colourful for readers!

Redcliffe foreshore © 2014 Andrew Sutherland

Redcliffe foreshore © 2014 Andrew Sutherland

One of the most rewarding mobile reception achieved with this antenna (above) was rare tropospheric ducting from Nouvelle Caledonie during September of 2013, whilst parked on an unobstructed peninsula (left). Granted, even with a 19 decibel masthead signal amplifier employed in this scenario, these signals were extremely weak! Nonetheless, the bottom line is that the folded dipole antenna offers an enormous potential for any receiving application.  (If reading about mobile mountain DX fun previously published on this blog, please remember that the half wave folded dipole antenna was employed on these ‘day trips’ without exception).

Harry's Hut © 2006 Rae Allen

Harry’s Hut (3.7 mi / 6 km W of ocean) © 2006 Rae Allen

Typical DX application with the folded dipole & Degen DE1121 at Harry's Hut © 2014 FM DXing

Typical DX application with the folded dipole & Degen DE1121 at Harry’s Hut (36 ft / 11 m ASL) © 2014 FM DXing

Harry's Hut © 2014 FM DXing

Harry’s Hut © 2014 FM DXing

In September of 2014, this antenna was used at the insect-ridden Harry’s Hut day-use area (above) situated on the western bank of the upper Noosa River. The folded dipole was hand held vertically by the ‘old man’ (dutifully assisting) a few metres above a park bench (inset, right) a short walk from the jetty and canoe landing facilities (below). As a contrast to the peninsula trip recounted above, nothing is notable about this stop! It is included merely as a typical illustration of mobile FM reception that may be ordinarily expected with this antenna (using a narrow filter modified Degen DE-1121 portable recording receiver).

Harry's Hut jetty © 2006 Rae Allen

Harry’s Hut jetty © 2006 Rae Allen

On this evening (during a ‘quick and dirty’ 15 minute listening window available) there was enjoyable jet-reflected scatter from community radio in Byron Bay (coastal S) to public radio in Rockhampton (inland NW) during flat spring conditions (zero ‘tropo’) comprising a scatter range of 390 mi / 627 km. Even the terrific country narrow caster from Bundaberg was audible.

These signals were NOT audible on the Toyota Prado factory double DIN CD/cassette radio, a quality Japanese DSP receiver. Due to superior design, any contemporary car radio typically exhibits greater sensitivity than its portable radio counterpart. For a portable radio to exhibit such out-performance in the field can be attributed to the use of the folded dipole antenna. Such a comparison seemed fair, since both the vehicular antenna and the portable antenna were operated at the equivalent height above ground. Adjacent channel interference was NOT a determining factor in reception.

Any inexpensive portable receiver that features an external FM antenna jack will accommodate a folded dipole (or loop) FM antenna. These radios include the CR-1100, DE-1103 (KA-1103), DE-1121 (KA-1121), PL-660, PL-606 or PL-310ET from the Tecsun manufacturing parent company. Others include the Digitech AR-1748 (bloggers suggest this may be manufactured by Redsun) or the Sangean ATS-505P, to mention a few current models. An antenna adapter (typically a 3.5 mm audio plug to PAL socket) is required. These are available from $4 from electronics retailers. Make no mistake… this writer believes that purchasing a portable receiver which accommodates an external antenna (and the use of a ‘homebrewed’ or commercially-made external antenna) is worth the extra effort!

What about a wire folded dipole?

Wire Dipole inside window © 2013 FM DXing

Wire Dipole inside window © 2013 FM DXing

The above observations are limited to the ‘metal version’ of the half wave folded dipole. It should be acknowledged that a twin lead wire version of this design is widely available. Whilst physically similar to the wire dipole (left) it differs from that ‘pure’ dipole design by having joined conductors and a 300 ohm impedance. Detailed information on the limitations of wire-based antennas appears in the table and references below.

Terk offer three amplified indoor antennas designed solely for FM reception. Amongst these models, the FMPro is based on a folded dipole design. It is weather proofed for use outside. Rival manufacturer Magnum Dynalab offer the SR-100 Silver Ribbon Tunable antenna.

ADVANTAGES DISADVANTAGES
Improved Voltage Standing Wave Ratio (VSWR) measurements at the terminals suggest best efficiency over most other simple antennas, including twin lead (ribbon cable) antennas. More expensive to buy than other simple antennas such as a wire antenna or Rabbit Ears.
Wider bandwidth than a single element dipole. (David Jefferies PhD suggests a sensible single dipole may exhibit a 15% fractional bandwidth). Although the folded dipole is still often classified as a narrowband antenna, the wider bandwidth may be considered a favourable attribute for reception on the FM or digital television bands. May be considered unsightly; beauty is in the eye of the beholder (or ‘beer holder’) of course!
Higher directivity than a single element dipole. Because of its directional characteristics, ideal for mobile reception to null strong, unwanted local stations. Due to higher impedance over a single dipole, requires a balun (usually supplied) if connected to a tuner with a 75 ohm coaxial input only.
Ideal for ‘mating’ with a quality masthead pre-amplifier if using a car receiver or component tuner. Unsuitable for air travel because does NOT fit in standard ports (suitcases).
Easily fits in the cabin of the average sedans (including in the boot). Depending on the size (a function of the wavelength) the copper version may be quite heavy (but effective exercise) if transporting to a summit or forest clearing.
Extremely durable; it will probably outlast one’s portable receiver! Significantly better durability than telescoping rod antennas such as Rabbit Ears.
Easily fits in a bedroom for indoor reception near a window.

As always, the writer has no affiliation with any retail merchant or product manufacturer mentioned. This entry is NOT intended to be construed as an endorsement of any particular product. Prospective buyers should carefully make their own enquiries according to their particular needs and circumstances.

The author wishes to acknowledge the valuable assistance of David in providing his feedback and personal observations of earlier drafts.

Altitude measurements performed with Celestron altimeter calibrated on-site. Calculations performed with Google Earth using ACMA KMZ data.

* Due to abnormally hot & active tropospheric ducting experienced during October & November, the aforementioned copper folded dipole DIY project will now be published (later than originally scheduled) during December in two parts. The writer apologies for the delay in publication. 


ANTENNA SUPPLIERS

D-lemp Communications Limited

Magnum Dynalab

Rescue Electronics Surplus

Terk Antenna Range


ANTENNA TUBING V WIRE EFFICIENCY

Holl_Ands

Rescue Electronics Folded Dipole


ANTENNA MATERIALS

Building antennas from everyday materials

Spark Fun United States

Tapes Online Australia


DIY DESIGNS TO BUILD WITH ALUMINIUM TUBING

Copper or aluminium foil tape dipole

Copper foil tape antenna

Stealth antennas made from conductive foil


DIY DESIGNS TO BUILD WITH ALUMINIUM TUBING

Folded Dipoles

Element diameter considerations


DIY DIPOLES TO BUILD WITH COPPER TUBING

Bruce’s dipole for FM band

Dave’s off-centre single dipole

850 MHz dipole for Wireless Internet

Nepaeric’s dipole for FM band

Single dipole for two metre band amateur radio 

VHF high band dipole for Digital TV

VHF high band & UHF dipoles for Digital TV


DIY ANTENNAS TO BUILD WITH COPPER TUBING

189 MHz yagi for Digital Television (includes dipole)

P2P micro-powered FM broadcast antennas used by uni students in Melbourne (2009)

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Is DXing a hobby only for the rich?

Since this was originally published, G1VVP has now freshened his perspective in Elitism in the Hobby which offers a comprehensive narrative on this topic. His original opinion piece inspired an entry of this blogger’s own. Further thoughts about the exclusivity prevalent in particular sports & hobbies have followed over the years; this piece originally from 2011 has been revised in December 2013 to reflect that progression. Please see also: Elitism in DXing: a succinct treatise

Stop ‘Helping’ Already is a 2014 narrative published by Todd Dugdale, KD0TLS. In this piece, Todd offers a brilliant insight into ‘mentoring’ practices. He also discusses what could be classified as exclusive marketing techniques, that is the apparent need to spent heaps of money just to try fit in. His refreshingly candid perspective offers broad applicability to all radio-based hobbies, not just amateur radio! 

Controversy keeps us alive. Tonight, we consider this question…

Is DXing a hobby only for the rich?

What distinguishes a ‘dabbler’ from an enthusiast? Is there a place for such elitism in a hobby which is declining in popularity for the X & Y generations? Take this example, and the hundreds worldwide that likely accompany it:

One young hard-core American enthusiast has yielded over 1,600 FM logs (that is, loggings of distant FM radio stations) using a combined VHF/UHF antenna and a rotator.

Does this mean his logs are NOT considered DXing because the antenna used is a combined VHF/UHF antenna, rather than an FM-specific antenna, like the APS 13 element antenna? Some enthusiasts cannot afford a rotator, nor specific antennas for VHF, FM and UHF. Notice the worldwide recession anyone?

 Do we live for controversy? © 2006 Luke Dorny

There is a very small minority of British enthusiasts who frequent hard-core DX forums insinuating that every modern day enthusiast is a fool. Perhaps this is NOT their intention; some enthusiasts may just be inarticulate when they express themselves. Everyone knows how easily written communication can be misinterpreted, and use of telephone and face-to-face communication are less problematic.

However, this blogger and numerous others tend to arrive at that conclusion time and time again. These enthusiasts are world renowned TV DXers. To fail to consider what they are saying would be like putting your head in the sand.

I think the word “DXer” is a bit too grand describe DXing with an aerial intended for totally the wrong frequency…One might as well use a telescopic aerial and be done with it. Possibly “Dabbler” is a better word than “DXer” with this sort of DXing setup. In days of old, one put an antenna up to cover the band required with a rotator if one wanted to DX weak signals properly from all directions.

But are opinions, such as the above quote, accurate? Are the ‘dabblers’ genuinely foolish with the equipment they choose to use? I would argue people are restricted in the hobby. Any hobby. Wouldn’t every renter put up a large antenna if their body corporate allowed them to?

Learning potential

What if someone wanted to venture into the wonderful world of FM DX but couldn’t even afford an indoor antenna?  Remember, this is a hobby that potentially teaches its enthusiasts about:

  • geography,
  • meteorology,
  • propagation,
  • radio markets,
  • overseas languages, current affairs & custom,
  • social networking with peers throughout the world,
  • radio electronics,
  • home maintenance techniques &
  • antenna theory.

…to name just a few of the learning outcomes enthusiasts might look forward to by pursuing DX!

Additional perspectives

Paul Logan is a relatively young enthusiast from Northern Ireland who is taking a stance against this elitism. Logan is NOT a ‘dabbler’. He is an amateur radio operator and world record holder who is actively promoting the use of portable radios through ‘Ultralight techniques’ to revitalize interest in the hobby. Today, Logan even utilizes the Whisper mode to conduct weak signal amateur radio contacts, a ‘bleeding edge’ digital mode favoured amongst the most progressive amateur radio operators.

When the Ultralight AM listening “craze” took off in 2007 thanks to American radio enthusiast Gary De Bock it was only a matter of time before the scaling back of our pursuit would be extended to the FM band.

Tecsun PL-606 with full length monopole  © 2013 Stephen Airy

G1VVP is a club founder and amateur who achieved transatlantic DX amateur radio contacts in 2010.

I get very angry when people take a ‘holier than thou’ attitude and make sweeping statements, dismissing those whom they feel are beneath them. Perhaps there is a reason: I know that ‘some’ DXers and radio amateurs get very upset if they have been missing out on exotic DX. They have invested large amounts of time and money to improve their setup so they become disillusioned to discover that someone down the road with a bucket and a piece of wet string has been hearing far more than they have with their lattice towers and stacked arrays. That is life!

Much stunning DX has been worked or received over the years on very simple equipment. We even have groups for those interested in ‘QRP’ or ‘Ultralight’ which present more challenges to the DXer. Many of these people go on to achieve ‘as much as’ and sometimes ‘more’ than others with elaborate equipment. Why? It could be because of the skill of the operator or because of dogged patience and a little bit of ‘luck’. While it may not ALL be attributed to luck, it is generally agreed that good fortune plays a significant part from time to time. in getting that exotic DX catch.

Elitism or common sense?

Are you an enthusiast your twenties or thirties? A teenager perhaps? What are you doing to stop the elitism infecting your hobby? Is the elitism just another example of bigotry and intolerance seeping into the social environment?

Perhaps you believe that elitism is good for the hobby? Is it a necessary part of every pursuit? After all, even beer drinkers can be tiresomely elitist, says homebrewer Barley Pop Maker in his hilarious satirical piece. Do you recognize yourself in one of his categories?

Perhaps it is NOT elitist to criticize receiving equipment that fails to meet minimum standards of functionality? For example, if one cannot receive a relative abundance of weak signals throughout the year on a portable receiver (probably one of the goals of a DX-based hobby), then criticism of using that particular method by a peer may NOT necessarily represent an elitist attitude, per se. Instead, it may represent an attempt to assist other enthusiasts into sustainable listening habits to avoid long term frustration. That potential frustration could ultimately result in an enthusiast quitting the DX hobby, which in contemporary times is becoming an increasingly difficult pursuit due to band congestion. Valuable talent is lost.

'Kill your Ego' at Burning Man © 2012 John Eikleberry

On the other hand, let’s be realistic about human nature. Status battles form inherently in pursuits that are NOT rewarded by income, but by achievements alone. Go to any sporting or hobbyist club to see this in action!

Politics within clubs

Problems that may exist in some DX enthusiast clubs tend to mirror the widespread problems with amateur radio clubs. As suggested in that excellent article (well worth reading entirely), if politics within any club over the long term is ‘sucking the fun out of’ the hobby, membership may constitute a waste of precious time. At worst, rather than learning anything new, time in a club may be spent arguing about trivial details!

If a ‘higher status’ peer ridicules others primarily because it is fun, the recipient is an easy target (e.g. new entrant or ‘lower status’ in the hierarchy) and the peer’s argument seems to be superficial and devoid of any logic… then perhaps elitism or even subtle bullying may genuinely be at play within a club or forum.

On-line forums might also just as be problematic for sensitive personality types. No one can deny that the world wide web (and in particular, forums) may facilitate a world of social interaction where ‘keyboard jockeys’ can taunt others without fear.

Be yourself

One can easily ‘go it alone’ in the world of DX without membership of a club or forum. The same applies to those  without access to expensive equipment or an acreage-sized rural backyard! Please don’t be afraid to be yourself (maverick or otherwise) in the hobby. As Oingo Boingo sang in the eighties, ‘there is nothing to fear but fear itself’.