Radio Antennas
What is an antenna?
An antenna is simply a conductive element that is designed to receive and often also transmit electromagnetic energy at radio frequencies. There are many different types of antenna designed for operation on specific frequencies or a range of different bands. Antennas designed for a specific frequency or single band are generally more efficient than designs that operate across multiple bands or groups of frequencies.
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The size of an antenna is related to its frequency of operation, larger antennas will generally operate on lower frequencies than smaller antennas, but there are some antenna designs that while being physically small they provide the electrical characteristics of a much larger antenna, these are usually less efficient than a larger antenna for the same operating frequency. Reduced size antennas are a almost always a compromised design where an acceptable balance between operational efficiency and physical size are required for a particular use or location where a larger antenna is not practical.
Mono-Band & Multi-Band
Multi-band antennas are very useful where there is a requirement to operate on multiple bands but there is not enough space to erect a mono-band antenna for each individual band. Multi-band antennas are always a compromise and can be very tricky to design in order to achieve acceptable efficiency across all the required bands. Usually one band will be a good impedance match for efficient operation and the others will exhibit a higher or lower impedance than required for efficient power transfer, as a result those bands will suffer with reduced performance.
Some multi-band antennas are specifically designed and constructed to be resonant on more than one band, most antennas of this type will provide reasonably efficient operation on those particular bands if is constructed and erected correctly. a fan dipole arrangement is one example of this type of antenna.
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A mono-band antenna that is resonant on the frequency of operation and is a good impedance match to the radio equipment being used will always be more efficient than a Non-resonant antenna, no matter what the marketing hype from the manufacturer or seller claims. Fancy marketing language and pretty looking diagrams help them to sell more products to unsuspecting buyers, but real physics always wins.
Buy it or Build it yourself?
Many people will simply buy a commercially produced 'off the shelf' antenna to suit their requirements while others will prefer to have a go at making their own antennas to find out what works best for them.
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Amateur radio operators are licensed to experiment with antenna design and construction. Indeed over the last 100+ years the amateur radio community has developed new radio communication technologies and many important discoveries have been made by amateur radio operators experimenting with equipment and antenna system designs, a few of these discoveries made significant changes to the way radio is used and caused the use of radio communications to advance beyond what was believed to be possible at that time.
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Some radio communication systems require that only commercially produced antennas certified for use on that specific system are allowed. Some types of radio only have fixed antennas or internally fitted antennas with no option to use an external antenna. The PMR446 standard in the UK specifies that only approved radios can be used 'as bought' and it is illegal to modify the radio in any way or connect an external antenna to these radios as they are specifically designed for license exempt use, they are for low power (500mW) short range operation only. It is illegal to use any other radio equipment on the PMR446 band that is not certified for use on that specific service.
Homebrew Construction
When it comes to using radio receivers there are many different antenna types available that are designed to optimize reception on a particular band or group of bands. On many receivers they have an antenna inside the receiver case or a telescopic antenna built in that is pulled out and extended for use and pushed back inside the body of the radio to protect it when not in use, these are often all that is needed to pick up relatively strong local stations. When radio propagation conditions are good it is possible to receive other stations from much greater distances but most of the time only stations relatively local will be heard well.
Some radio receivers will also have an external antenna socket or other method of connecting an external 'long wire' antenna to help receive more distant stations across a wide range of frequency bands.
Most shortwave radio receivers have connection points for a ground wire and antenna wire allowing the user to string up a very simple antenna consisting of a random length of wire to listen across a very wide range of bands. Anyone can experiment with different antenna designs for receiving radio transmissions.
You may find that a particular length of wire, a conductive metal pole or some form of loop arrangement works best at Your location. Try various different designs to discover what works best on Your particular receiver. Some antennas will 'pull in' more unwanted noise than others and some radio receivers have better selectivity than others, this improves the reception of a selected station while reducing all other signals it is receiving to give You a cleaner, better sounding reception.
G7FEK multi-band HF antenna
The antenna shown above is my own homebrew version of the G7FEK antenna design. This was built entirely from scrap materials so was a very low cost antenna project. Although it looks quite large on the photo it is actually very compact for an HF antenna that works well on the 80m amateur band. It is about 47ft (14.4m) end to end across the top horizontal radiating elements with 25ft (7.6m) of vertical twin conductor made with scrap copper wire. The vertical twin-conductor section is fed from the bottom with both conductors joined at the bottom feed point. This means the vertical conductors are used as linear loaded vertical radiators and the whole antenna is effectively a pair of Marconi inverted L antennas nested back to back.
The antenna itself is a 'T' shape, the poles and guy ropes at the left and right of the photo are only there to support and tension the G7FEK so that its feed point at the bottom of the vertical twin conductor section is about one foot (30cm) above the ground. The original design of the G7FEK includes balancing counterpoise wires below the antenna connected to the outer braid of coaxial feed line at the antenna feed point. I do not use counterpoise wires on my G7FEK as I have found it works better for me at this particular location using a long 3/4 inch (about 22mm diameter) copper ground rod driven about 7ft (2m) into the ground with a chunky earth wire connecting it to the outer braid of the coaxial feeder at the feed point.
As with all of my antennas I use common mode chokes on the coaxial feeder to stop common mode currents flowing back down the outer shield of the feeder and radiating, You only want the antenna to radiate, not the feed line. I also have common mode chokes on coaxial feed lines before they enter the shack, this not only helps stop the feed lines radiating which causes signal loss and interference but it also keeps common mode currents from flowing back into the shack as that can cause a range of problems.
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In direct comparison 'A' - 'B' swiching tests against other multi-band HF antennas I have found the G7FEK to be a good low noise receiving antenna with very good transmission characteristics. There have been a few other antenna designs that were better on one or two bands but they were all much larger, between 95 and 270 ft long, and they were all deployed at greater height than the G7FEK.
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For anyone with limited space to put up an 80m antenna I can recommend giving the G7FEK a try.
if You have less than 47 ft to string up the top horizontal wires You can hang it up so that as much as possible is in a straight line but part of the long leg of the horizontal radiator turns 90 degrees to a third support. This will change the shape of the antennas lobe pattern and may slightly reduce efficiency but it will still allow You to have a lot of fun on most of the amateur HF bands.
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I have used this as my primary HF station multi-band HF antenna for just over three years of low power 'QRP' operation and have been very happy with its performance with over 18,000 confirmed QSOs across 150 countries in the log using my primary MM7WAB callsign at the time of writing this article. I also used it to work over 2,000 stations in 82 countries using the callsign MQ7WAB during the month of June 2022 as part of the Queen Elizabeth II Jubilee celebrations.
The fact that I live in a rural location 700ft up a hill and am lucky to have excellent ground conductivity due to high ironstone content in this area are certainly contributing factors in my success with my HF station.
Two element Moxon for 2m VHF
A VHF Moxon is another very cheap and easy to build project. This one was constructed from repurposed scrap materials, a wooden curtain pole, an old wooden coat hanger, some old 2.5mm Red & Black Twin-and-Earth cable with external cable sheath stripped off to provide two lengths of insulated copper wire for the driven element and reflector. I also happened to have a short length of coaxial cable left over from another project that already had a BNC connector fitted so that was checked with a meter and then soldered to the two halves of the driven element. Total antenna cost was Zero and took less than an hour to build.
If You had to buy the bits the connector would probably be the most expensive part unless You used a long coaxial feeder cable to use the antenna at a fixed location or on top of a tall pole.
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The red reflector element is the back of the beam so the primary forward lobe is in the direction of the black element. Unwanted noise or relatively strong QRM can be can be reduced and sometimes nulled out completely off the back of the beam. The forward lobe is quite wide so You don't have to be too critical with azimuth beam headings, simply sweeping the antenna back and forth it is easy to find the peak strength of a single station. This also makes it quite good for calling CQ with eight 45 degree steps easily covering a full circle from a summit or other elevated location.
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I use this antenna for Portable handheld working when walking up in the hills and have carried it tied to the frame of my bike or lashed to my backpack when travelling light with just a hand held radio. It is quickly detached from the bike or backpack and deployed as a hand held beam when I find a nice location to stop.
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The coaxial feeder tail has a BNC type connector making it quick and easy to hook up various 2m radios fitted with suitable adapters as required. I often use this Moxon beam with a hand held 2m radio lashed onto the wooden antenna handle by the radios belt clip. It is then operated with an external speaker mic plugged into the radio making it easy to handle, one hand holding the antenna and one hand for the mic. When using hand held radios on this Moxon I usually run 1W low power and have made many QSO's at ranges beyond 50 miles from locations up in the hills. Running low power has the added bonus of making the radios batteries last longer.
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I have also used this antenna to very good effect by sliding the antenna pole inside a length of PVC electrical conduit lashed to the bike trailer. (see photo above) I then use a short length of coaxial feeder plugged into the Kenwood TM-701E running off a small SLA battery on the bikes rear carrying rack or a car battery housed in the bike trailer for a day or more of operation. With the TM-701e on low power, 5w, I have had very good results with several contacts well over 100 miles.
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I built this Moxon based on calculated dimensions from the Moxgen software and then made slight adjustments to the gap spacing between the elements to dial in the antennas center frequency to 145.450 as I primarily use it for working in FM mode.
Other antenna projects
I have also built and experimented with a wide variety of other antennas at various locations:-
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Half Squares, (HF 40m, 20m & 17m)
Delta loops. (horizontal & vertical) (HF,VHF)
Bifilar 'flag' (HF 160m, 80m, 40m & 30m,)
Slopers (HF various bands) Often use center fed dipole as sloper when fixing points limited /A or /P
Marconi inverted L`s (HF various bands)
LOG 'Loop On Ground' mostly RX but also used LOG on 160m, 80m & 40m for NVIS short range operation.
Base loaded, center loaded and distributed load verticals (HF,VHF,UHF)
Phased Arrays. Vertical and Horizontal. (HF,VHF)
Trapped dipoles (HF 80m, 40m & 20m)
Double-Bazooka coaxial dipoles (HF 80m,40m,20m & 17m)
Vertical co-linear arrays (VHF,UHF)
Horizontal quad loops (HF/VHF outdoors and in attic space)
Fixed and portable NVIS wire antennas (160m,80m & 40m)
Horizontal V's (HF various bands)
Horizontal OCF 'Z' (HF) : Note; feedpoint at 42% from end of wire worked best at my location.
End fed (HF) : Note. OK for RX, terribly inefficient on TX.
534ft 'meandering wire' forming a pair of horizontal loops (160m)
256ft 'twisted pair' wire OCF (160m - 6m) with two 130ft+ counterpoise wires.
132ft 9in enameled copper wire OCF dipole (80m CF 3.760MHz with feedpoint 42% from end)
2 ele Moxon rectangles (17m,6m,2m,70cm)
2 ele Yagi-Uda wire array (80m) this was strung up in trees at edge of a forest for a summer season.
3 ele Yagi-Uda wire array (17m)
5 ele Yagi-Uda array (2m)
8 ele Yagi-Uda array (70cm)
2 ele Quad (17m,10m and 2m)
3 ele Quad (2m)
Hexbeam (17m, 15m, 12m & 10m)
Folded dipoles (monobanders from 80m - 70cm) also used as driven elements on some Yagi-Uda arrays.
A selection of experimental magnetic loop STL antennas.
Turnstile (2m, 70cm) 'standard' using phased dipoles and 'high gain' using phased Moxon beams.
Curtain Array (VHF) 3 bays high X 4 bays wide.
Various patch, slot, PIFA and multi element arrays for mobile, mesh & fixed data links (700MHz - 5GHz)
Assorted arrays, quads, bi-quads, parabolics & waveguides for WiFi mesh & IP data links (2.4 & 5GHz)
Assorted small RDF arrays.
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Increasing antenna system gain.
More gain is not always a good thing.
Fitting an antenna with higher gain to a radio transceiver will improve both transmission and reception of signals, thus providing an extended operable range or a greater signal strength over the same range, but like most things, when adding an antenna system with more gain there is also a catch !
When using the same RF power output level from the radio equipment Increasing antenna gain will increases the level of power transmitted from the antenna, There comes a point when the antenna output ERP (Effective Radiated Power) becomes greater than the limit of power output that is legally allowed, so the power output of the radio is reduced to keep the ERP level within the legal limit. No real problem there but on receive it is an entirely different matter.
A higher gain antenna system will not only improve reception of the signals you want but also the signals you do not want. In locations where there are numerous other devices on the same band or high levels of interference, the receiver can be 'swamped' by unwanted signals making it incredibly hard for the receiver to pick out out the signals you do want. Such cases require careful balancing of antenna gain, transceiver sensitivity and transmission power to achieve acceptable levels of stability and performance in a noisy environment.
This is exactly what I did with our WISP (Wireless Internet Service Provider) 2.4 GHz microwave broadband radio link a few years ago. With everything well matched and adjusted for the best SNR (Signal to Noise Ratio) performance was quite good for a while until the nose level started to rise again.
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I experienced serious interference problems on our 20km 2.4 GHz ISP link and after some considerable time and effort trying everything possible including deploying a parabolic dish reflector with higher gain to improve the link stability, adjusting power levels to keep the transmit ERP correct and employing bandpass filters to reduce the unwanted out of band noise level. This turned out to be more of a curse than a blessing in some ways.
The high gain dish was installed and the impedance almost perfectly matched to the data link radio and RF power output of the radio was finely adjusted to transmit just below the maximum legally allowed ERP from the new antenna. Even with very careful adjustment of a new multi-stage sharp cutoff bandpass filter providing very high attenuation of unwanted signals it still received a lot of RF noise!
The radio unit was bombarded with unwanted signals that effectively reduced the Signal to Noise Ratio to levels that were so bad the radio was struggling to pick out the required signal from the remote tower 20km away as it was swamped by the high levels of interference.
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The noise problem was only solved by accepting defeat and moving to a quieter, less congested band.
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The 2.4 GHz equipment was replaced with a more powerful system operating at the upper end of 5 GHz band. The link stability and bandwidth improved greatly, performing well most of the time.
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The hidden issue with going up in frequency changing from 2,4 GHz to the 5 GHz band soon reared its ugly head in the form of attenuation and signal scatter in poor weather conditions.
The link worked very well with low latency and very little packet loss in fair weather, but when it rained the link packet loss and jitter started to have an adverse effect on performance.
With our house being located 700ft above sea level up a hill in South West Scotland and the 5 GHz high gain parabolic dish reflector mounted on a pole 25ft above the ground it worked very well in good weather but when the weather turned poor, signal scatter became a real issue. Packet loss became very high and the link stability suffered badly with severe bandwidth limitations when low cloud or mist shrouded the village as the small water particles in the air scatter and attenuate 5 GHz signals. When we had heavy rain, thick fog or snow the link would quickly become very unstable and frequent disconnections would occur.
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This was the best we could manage with existing commercially available equipment at the time and with no other means of getting an internet connection to the house this was my primary internet link for a few years until the cellular networks were extended into the area providing the option of a 3G mobile data connection service that was slower but much more stable. We kept the 5 GHz link as this was faster than the 3G service in good weather and used the 3G mobile data service as a lower bandwidth backup for when the weather turned poor and the 5 GHz link became unstable or slower than the 3G service. This was setup as a fail-over system that would drop back to 3G data modem link when the 5GHz link dropped out or became slower than the 3G service.
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The 3G data service was improved after several months when two cell tower sites a few miles away were upgraded to 4G providing a data connection with higher bandwidth and lower latency . This was used as a backup fail-over service for when the 5 GHz microwave data link was struggling.
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Fiber to the premises takes over, but cellular radio data link is still needed.
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We now have high speed fiber to the premises which is very fast, low latency and reliable so the 5 GHz WISP was been decommissioned and the hardware used elsewhere for shorter range data linking.
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The fiber broadband is great compared to the old radio link as it does not suffer with signal propagation issues in bad weather but, when we have mains power cuts, which has happened several times over the last few months, and the fiber service goes down, the modern VOIP telephone service also fails as it does not have power supplied to subscribers telephone apparatus from the exchange as the old copper twisted pair telephone lines did.
This is a major leap backwards in telephone technology as it means emergency calls can not be made during a power failure using modern VOIP telephone systems.
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When fiber ONT (Optical Network Terminator) or router at a subscribers premises are powered down the phones have no internet connection and do not work so in any emergency that drops out the power the VOIP service will be of absolutely no use to call the fire service, ambulance, police or anyone else for help. Simply providing the ONT and router with alternative 12v battery backup power will keep the units at the subscribers premises running, at least for a few hours, but the data and VOIP services will still not work if the equipment on the head end of the fiber also has no power due to the domestic mains power supply being down at the exchange. So having a backup cellular data link could save lives !
Each time the grid mains power supply has gone down over the last six months or so the 4G cellular network radio data link fail-over still has been very handy as a battery powered backup connection.
This allows everything on the LAN and WLAN Mesh network that runs off battery power to still be online when the village has no mains power. Until the cellular network tower site loses its network link feed or the cell tower site backup power runs out then the only means of communication is using my amateur radio equipment running off car batteries.
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I still use some 2.4GHz and 5 GHz links and Mesh networking technology to tailor my WLAN coverage to suit my needs locally depending on where I need to extend an IP data link to. These systems all use relatively low gain antenna systems to reduce received noise and achieve a good SNR optimised for short range linking between low power devices.
East Ayrshire Radio Society
Ayrshire, Scotland.
© 2022 MM7WAB Hairy Paul