DIY Archer VHF NAV Antenna

There is a popular antenna available such as this one from Aircraft Spruce, but to be honest it seems very simple to make, and the shipping would be astronomical from the US for such a light, but large item. They don't sell them as kits unfortunately. 

Here is the basic plans, straight out of the AeroElectric Connection:

Now, I know NOTHING whatsoever about antennas - or i should say, i KNEW nothing about antennas before beginning this project. I am not even totally sure i will want a VHF NAV antenna in the aeroplane at this early stage (who knows if there will even be any ILS's left when i am finished), however if i were going to have one, it was a lot easier to run the COAX now, plus i needed to install nutplates etc etc. Since i have some time (my fuselage is waiting for a ship to be available), i decided i may as well begin my antenna journey. 

Antenna Sizing

The obvious option is to simply copy the plans exactly and build the antenna as it. However, the -14 has much larger wingtips, so i sought some advice online on how i could increase the size of the antenna. 

Here is a link to the discussion. 

In summary, i needed to keep the width of the 'arms' the same, and the size of the 'gamma match' triangle the same. The 'ground arm' - aka the bracket which holds it to the wing, needs to be electrically grounded to the airframe but it's length didn't matter. The 'span wise' arm - aka the arm leading away from the end of the wing could be as long as a wanted, however if i increased the length of this, i would need to decrease the length of the 'chord wise' - aka the trailing arm. 

In order to work out the dimensions and the correct angles, i took some measurements and used a cardboard template. This gave me the basic outside dimensions, and correct location to allow the antenna to nest into the centre seam of the fibreglass wingtip. 

This is where the outer arm of the antenna will eventually rest. I will use a piece of tubing cabletied to the antenna arm so it nests nicely in the finished wingtip.

The other discussion, was on the 'capacitor' construction - this is the little piece of aluminium which the centre of the COAX connects to. This 'floats' over the little piece of Bakelite which is the 'dielectric' in the capacitor. Now, i have no idea how this works, but it does! Somehow the 'transmissive' properties (aka the 'permittivity') of this piece achieve the 'gamma match' part, and tune the antenna to the COAX impedance. It is important that this is built the same. 

Here is where i ran into problems - the plans call out 0.032" Bakelite as the 'dialectric'. I could not get a hold of this, however had some 0.040" Fibreglass sheet. The properties of these 2 materials was apparently similar, however the thickness would cause a problem. 

Essentially, the amount of 'capacitance' created by the device, is based on the distance apart of the upper and lower aluminium plates, as well as the 'permittivity' properties of the materials. Ignoring the effect of the properties of the material, the thicker 'dielectric' would result in a lower capacitance - which would necessitate a longer upper plate. The original plans called for a 0.032" x 3.25" Bakelite strip, and a 2.5" long upper plate. Since i was using a 0.040" x 3.25" fibreglass strip, i assumed this meant a lower capacitance, so i lengthened the upper strip to 2 11/16" - giving me some wiggle room to shorten it later. 

Similarly, I left the 'chord wise' aka the trailing arm, very very long to give me the room to trim it later during the tuning process. 

I made up some drawings so i had some idea of what i was building, and these game me some templates to build on. 

This is a side on view of the 'dielectric gamma match capacitor' - i drew it out like this as i had some 4-40 holes, and some 10-32 holes and i needed to ensure proper edge distance. 

Yes, my aeroplane will have some RC aircraft parts in it! 

Here i am cutting the fibreglass plate to size.

And marking the holes for the nylon and steel bolts.

I match drilled the plate to a longer piece of aluminium which would eventually form the bottom 2 plates. 

Then match drilled the upper plate. 

The lower plate then gets split. This gap was specified in the plans to an exact dimension. However, i had to make sure the hole was perfectly centred, as a 4-40 nut had to go on this side and needed to be electrically isolated from the bottom plate. This took me 2 goes to manage to get this perfect!

The plans call out a 5/16" gap here - or 0.3125" - i think this is good enough!

Here is the final dimension of the upper plate - this was made long to allow me to trim it shorter later when tuning the antenna. 

Once the 'dielectric' arm was made, I could match drill it to the other arms, to make a 60 degree triangle. 

The whole assembly was then riveted together using aluminium AVEX pop rivets. I used the Dybro RC Aircraft Wing bolts and regular nuts to hold the dielectric together, and 4-40 Dsub hardware the temporarily put the COAX connector together. This was replaced with 4-40 CAD plated screws and locknuts for final install of the COAX (as i assume it would have to be removed for tuning). 

Here you can see how the nylon bolts sandwich the fibreglass 'dielectric' together, but don't electrically connect the top and bottom plates. The centre conductor of the COAX is then electrically connected only to the upper plate. The shield of the COAX is connected electrically only to the lower plates (and therefore ground).

I used a piece of wire would around the shield. This was soldered together, careful not to melt the centre conductor insulator, then a piece of clear heat shrink tube was used over the top.

I then crimped on some #4 terminals. 

A BNC bulkhead connector was installed on the end rib.

This hardware was replaced with proper hardware later. 

Getting some help

Next up i needed to tune the antenna. I enlisted the help of a local HAM radio club - Melbourne Electronics and Radio Club - VK3FSK - their website can be found here. 

A lovely gentleman called John Chenoweth came to my rescue, and over the course of a few days, lots of emails and a number of long calls, gave me a lot of advice and taught me how the antenna that i was building actually worked! (Well he tried his best to explain it anyway to a dumb pilot). 

One thing he did suggest was that any surface corrosion would severely effect the efficiency of the antenna and i should try and protect it if i can. So i decided to use some Tempo rattle can primer on the assembled antenna. I made sure to keep any primer off the inside surfaces of capacitor which were touching the 'dielectric' - aka the parts which were touching the fibreglass, and also kept some spots clear for the mounting bolts, and COAX connect points - to ensure a good ground. The antenna was installed using some K1000-3 nutplates - the AN3 bolts in 3 locations on the end rib. 

I checked the shield of the COAX at the wing root was connected to ground, and that the centre conductor was not - making sure there were no shorts.

Tuning

One fine Saturday afternoon, I had the pleasure to welcome John to my shop, who came with a lot of antenna knowledge, an hour or so of interesting antenna theory and a digital antenna analyser - a RigExpert AA-200

The basic principles (if i learned anything from John at all!) is that an antenna (including it's installation, ground, COAX etc) will resonate naturally at a particular frequency. That is, it will take in energy and reflect energy. There will be a particular frequency with which it will take in more energy than others - this is it's resonating frequency. This antenna is a 'quarter wave dipole' antenna - meaning that the frequency it will resonate at will be 4 x it's length (i.e. the length of the antenna is 1/4 of the wavelength of the resonating frequency). 

The efficiency of the energy usage is measured using "Signal to Wave Ratio" - SWR. This is a measure of how much of the received energy makes it to the radio, and how much is reflected back away from the radio. In a perfect world, a SWR of 1 would mean that 100% of the energy is being received. The Archer antenna on Aircraft Spruce state they all have a SWR of 2. 

Knowing this - we are striving for 2 things in the tuning process. 

1. To have the antenna naturally resonating at a frequency with which we can do useful work - in our case, we are wanting to use this antenna to receive VHF NAV signals which are in the range of 108 - 118 Mhz. The signal is horizontally polarized, so the antenna needs to be installed in the horizontal plane (in our case, horizontally off the end of the wingtip). We want to therefore tune the antenna to resonate in the middle of this band. An ILS frequency is around the 110Mhz mark, so we will aim this this. We adjust the resonating frequency by altering the length of the antenna.
   
   
2. The SWR needs to be low enough so the antenna is efficient. We want to aim for it to be lower than 2, as this was the benchmark for a commercial antenna.  Ideally, we want the lowest SWR to occur at the planned resonating frequency, but also that the SWR across the useable frequency band is also low (below 2). 

Given this explanation, the plan was to install the analyser and have a first look. We were expecting due to the long length of the antenna (deliberately) that the resonating frequency would be very low. The antenna was in the 35" length mark, whereas the commercial antenna has a length around 28.5". In fact we can do the math on the antenna length of 35" - it should be around the 88Mhz mark. 

We connected the analyser, and sure enough - it was resonating at around 81Mhz. In the image below you can see a graph - on the vertical axis is the SWR. The horizontal axis of the graph is centred on 81Mhz (81,000 kHz) - as shown by the little triangle. The displayed graph has a range which is 6250 Khz less and 6250 Khz more than the centre. The graph shows that at 81Mhz, the SWR is fairly useable at 1.5 - however, the steepness of the graph shows that in reality, if we are assuming a SWR below 2 is good, the useable range for this antenna, at this length, is only maybe 80-82Mhz. But we have a start! 

The next step was to start trimming off the end the the 'radial' arm of the antenna - in our case the 'chord wise' aka trailing arm of the antenna. We trimmed it off in an iterative process - starting off large and dialling in the frequency as we approached the range we wanted. 

The above shows the iterative trimming of the antenna. 

The below results are half way through the process. The graph is centred on around 110Mhz, with a range of +/- 12.5 Mhz. You can see a there is an area of the graph to the left of the centre, where the SWR is below 2. This is in the range of approximately 100-112 Mhz. Some more trimming is required, but we are going in the right direction. 

And here you can see the results when we stopped trimming. The graph shows that the lowest SWR of 1.5 occurs at the 111.3Mhz frequency. There is a useful band below the 2 SWR mark which is quite wide - in fact, perfectly 108-118Mhz range (see images below).  

The little triangle at the bottom shows the lower end of the 'below 2 SWR' band is about 108 Mhz..

Here is a printout of the results in PDF format. 

So in the end, we managed to tune the antenna without having to touch the plates of the capacitor at all. We were expecting to be able to tune the antenna as per the above process, however see a graph where the lowest SWR was higher than 2. If this had occurred, we would have had to adjust the length of the top plate of the capacitor. In our case however, i think we got lucky. The extra thickness of the fibreglass dielectric meant the top plate needed to be longer. And i managed to fluke to the right length. 

So how long was the antenna in the end? - 28.5 inches or thereabouts! Exactly the same overall length as the original plans. So for anyone ahead of me, if you extend one arm, shorten the other such that the overall length is close to 29-30". You will be much closer to the mark when tuning that we were, but we got there in the end! 

The finished article.

A big thankyou

A massive thankyou to John Chenoweth from the Melbourne Electronics and Radio Club for your time, explanations and assistance. I have learned a LOT!  - and that's what building an aeroplane is all about!