Using an aluminium mast as a 1/4 wave vertical?

Equipment Antennas
1

I’m looking for a cheap base station vertical antenna for SOTA chasing and some DX on 20m and/or 40m.

For portable use I have a fibreglass fishing pole with a ¼ wave element running up the inside. This seems to work very well portable, although the pole is a bit flimsy (not really any good for a permanent fixture). I figured that for a base antenna I should try to replicate this with a more sturdy pole.

Looking at the prices of fibreglass masts long enough to run a ¼ wave element for 40m, I’m looking at over £200 if I buy new from a fairly well known ham radio retailer (I might as well just buy a factory made mono-band or multi-band vertical and be done with it).

I already have several sections of 5 foot aluminium mast (the type which slot together to make a portable/temporary mast), although I believe that anything conductive (such as aluminium or steel) would de-tune the antenna.

That led me to wonder, CAN I USE THE MAST ITSELF as the vertical radiating element. After all, there’s nothing cheaper than what I already have!!! I believe that some commercial broadcast stations do actually do this?

In other words, can I trim the aluminium mast to a ¼ wave at whatever frequency (either 40m or 20m) and bury a bed of radials around it?

I can see two potential problems with this idea:-

Firstly, aluminium isn’t as conductive as the copper antenna wire that would normally be used for this sort of thing. I wonder if that could result in an inefficient antenna, or make it difficult to tune?

Secondly, the mast (which is effectively the vertical element) will need to be insulated from ground both at the base and on the guy wires. I also presume that metallic guy wires could have a coupling effect and de-tune the antenna? So I guess that I would need non-conductive guy wires/ropes?

So for you antenna gurus out there, I have the following questions:-

-Will it work, or is it a really stupid idea?

-How efficient (or in-efficient) will it be? Generally copper wire is preferred due to its better conductivity), so I’m wondering if using aluminium will make it inefficient? Having said that, many of the big antenna manufacturers do use aluminium for their antennas.

-Has anybody on here already tried this?

It seems like a really cheap and easy way to get a vertical up and get on the lower bands without taking up too much space but I would have thought that more people would already be doing this if it was that simple. There must be something that I’ve overlooked?

2

Firstly YES you can use the metal mast as the antenna. As long as everything is insulated from it and use nylon guy ropes not metal ones.

Not directly answering your question about building yourself but … If you can still find someone making and selling them, take a look at the SRC X-80 80-10m vertical. Orginally from Snowdonia Radio Company it’s a non-resonant vertical with an UNUN at its base and needs an ATU in the shack, that being said it works fairly well for a multi-band antenna. Especially one where radials are not mandatory (they will help but it will work without them).

73 Ed.

Update - here you go - 60 Quid from a UK supplier:

3

Just to expand on this and fill in the gaps a little bit on what I am actually trying to achieve………

I suffer from a significant noise floor at my home QTH (currently S9 on 40m & S7 on 20m), making operating from home virtually impossible.

I own around 3 acres of farm land which is approximately 45 minutes drive from my house, where the noise floor is about S0-S1 (basically only natural atmospheric noise).

I had the idea of setting up a remote station at the farm. I’ve got a couple of RemoteRig units which are working a treat with my Yaesu FT-857. The next step is to set up a 4G link to the farm for remote operation, box it all up so that it’s protected from the elements (military ammunition boxes?), power the whole lot with a solar panel and figure out what antennas to put up.

Unfortunately it’s classed as agricultural land so no chance of getting planning permission for a tower, as much as I would love to install a large lattice tower.

Basically anything I install needs to be “temporary” & fairly discrete. Partly to keep the planners at the council off my back but also for security as it will be unattended and I don’t want to advertise the fact that there is expensive radio equipment on site with a massive antenna farm!!!

I figured 1.5 inch diameter aluminium mast is fairly low profile, especially across the other site of the field where it should blend into the trees) and I should get away with that without drawing too much attention.

I’m undecided on what to do for the lower bands as I also want 80m & 40m NVIS capability for SOTA and other stations around Europe.

I have a duel band (trapped) dipole for 40 & 80 which I could mount fairly low to the ground. The height of an NVIS antenna seems to be a topic of much debate on various forums I have read but I believe that around 5 meters should work will for NVIS.

Alternatively I could put a full wave loop (a so called “sky loop” antenna) at about 5 meters above the ground. I reckon I may even be able to get a full 160 meter loop in there, which I should also be able to tune to 80 & 40 (it would be nice to have 160m as well). Perhaps not quite so discrete though as it would need 4 support masts (unless I could use some trees).

A more discrete option might be to just string a long wire across the entire length of the field between a couple of trees (although I’ve always found long wires to be a little fussy and not as good as a dipole or full wave loop).

I know it’s digressing from the original question but just to fill in the gaps and elaborate on exactly what I am trying to achieve.

4

NOW you’re talking!

My main antenna here is a 40m Skyloop at about 7-8m high. I tried to fit in an 80m one but it had so many bends and such in it, that it became inneficient. If you are looking for low noise and a realively low take off angle for DX, the Sky-loop is a great choice. It will run on many bands - my 80m loop worked fine on 60m without an ATU but needed an ATU for the WARC bands (except 30m that it just didn’t like). By feeding the loop with ladder-line and having a switch in the shack, I can change the antenna from a 40m loop to a top loaded 80m vertical which works well-enough on 160m as well. You may not know it but the loop is actually directional when used at multiples of its resonant frequency - so my 40m loop on 20m has a little gain in the direction of the opposite corner from the feedpoint. By the way a loop doesn’t have to be square. The more area you can get inside the loop, the higher gain it will have on all bands. Hence the best shape is a circle but square, oblong L-Shaped or whatever - they all work. It is a very forgiving antenna and of course a very cheap one to build.

The feedpoint will be between 150 and 200 ohms at the height you quote, so you can either feed with a 1/4 wave long 75 ohm Q-section or a 3:1 or 4:1 BALUN (Yes 3:1 BALUNs do exist). The Q-section of 75 ohm coax is the cheapest option and does NOT restrict the bands that the antenna will work on. On an 80m loop, the 75 ohm coax is 1/4 wave long at 80m allowing for the velocity factor of the coax and the antenna will work on 80,60,40,20,15 & 10m without an ATU and on the other bands (except 30m) with an ATU.

Checkout my documentation here: 40 metre Horizontal Loop Antenna. | DD5LP / G8GLM / VK2JI blog

If using those aluminium masts to support the antenna - run at least a metre of nylon rope from the top out to the wire on the loop. The same with the tree supports. Alternatively, do as I do and use fibreglass squid poles and then add plastic loops to their tops to run the wire through.

Yes you can use some trees as supports as the loop is horizontally polarised, the trees wont cause an issueas long as the wire is about a mmetere away from the (damp) branches. On the other hand trees can reduce the effectiveness from vertical antennas through absorbtion.

I suspect you most difficult problem to solve with a remote station will be to activate an ATU when needed as the FT857D doesn’t have one built in.

But if the bands you want to use are among the ones where the loop has an SWR under 2:1 you wont need a special remote ATU.

73 Ed.

P.S. on the home noise floor point - at those levels the noise is quite likley to be coming from your own home - I presume you’ve tried to trace the noise? Most likely culprits will be a switched mode power supply PC or solar panels. Connect the FT-857 to a 12v battery and then turn off the master power switch to your home and all battery powered computer equipment (Laptops tablets etc) - if the noise goes away - turn on the power circuits and PCs one by one until the noise comes back. Then find the power unit and deal with it - ideally with a sledge hammer!

5

Hi Ed,

Already tried running from battery and killing all of the power to everything in the entire house at the main circuit breaker board. Unfortunately this made absolutely no difference. I’m reasonably confident that its not coming from my house.

No solar panels on my house, but several of the surrounding houses (within 100 yards or so) do have them.

As I said, I live on the edge of a large estate, so the noise could be coming from any of probably over 1000 houses on the estate. Most likely noise from multiple houses in all directions combining together.

I think the only viable option that I really have (apart from sabotaging the local sub-station which will get me into mega trouble with the law!) is to get the antennas away from the estate (hence using the remote farm location).

The ATU tuning is a bit of a non-issue to be honest. There are two very viable options for that:-

  1. As you said, use resonant antennas, then you only need to switch antennas remotely (there are internet controlled coax switches available for that).
  2. The FT-857 has a function to control an ATU via the cat port for certain antenna tuners which allows you to activate the tuner through a quick access menu. I’ve not tested this yet (as the tuner is on order and should be delivered by the end of the week) Apparently the external tuner for the FT-897 (the AT-897 if I remember rightly) also works with the FT-857. There are also a couple of aftermarket tuners available which can be controlled remotely via the cat port on the 857, so I don’t think that will be a massive challenge.

I think my biggest challenge is security…in other words preventing some opportunistic piece of low-life scum from nicking my radio!!! Hence why I want to keep the setup fairly discrete & low-profile.

6

I hope this is still interesting for others reading this thread - perhaps we should move it off on to private messaging? (or simply emails?) - In any case one factor that comes to mind. If you are not intending to have rotatable antennas at the farm site. You could keep the transmit station where you are and only locate a cheap receiver out at the remote site. Then you don;t have to worry so much about an ATU.

Just a thought.

That AT857 - is that the LDG one or do Yaesu do one? In any case as you say if it’s controllable via the CAT port, it should also be controllable remotely.

What will you have in the shack? do you kep the head of the 857 in the shack and the body goes remote, with a hardware remoterig box at each end, effectively extending the RJ45 cable?.

You may wanty to look up the other thread on this reflector as a Spanish SOTA chaser has just done the same as you are planning - let me see if I can find the thread and then update this post, so you can read what he did.

73 Ed.

Here you go:

7

Ed,

All interesting as you say. Indeed I have read the other thread to which you refer with great interest.

I did wonder about splitting this off into a separate thread as we are steering wildly off course from the original question of whether an aluminium mast would make an efficient vertical antenna.

I’m not sure how to split the thread (it might be that only moderators can do that).

I think maybe we should start a separate thread specifically for this topic, or as you suggest, discuss it via private email if you want to drop me a line:-

Jamesday5 AT virginmedia.com

Obviously I’ve changed it slightly to prevent my email account from being spammed. I’m sure that you can figure out how to make that email address work!!!

8

Are you adept at climbing trees I wonder? Trees are excellent supports for coax fed wire antennae - the off centre fed dipole is popular and a better all rounder than the old chestnut G5RV. YOu could also experiment with a wire element vertical or an inverted L antenna activually inside the tree branches (ground radials needed to be effective). Some call the OCF a Carolina Windom. I am using both these antennae in a beech tree which is easily climbable, works for me. I have a 7m HD fishing pole in the top of the 40 feet beech tree secured by velcro straps and tywraps “Heath Robinson” stykle, a wire runs from a ground insulator to the base of the 7m pole where it goes inside - no balun. It comes out of the top to a pole on my house 70 feet away. Very effective as a 1/4 wave on 160m (No ATU) and 60m (ATU required) - these are the bands where it is of most use to me. At present I have around 1200 feet of radials, mostly in the lawn, with more to go in as time goes by. It makes a great difference to the efficiency of the inverted L.

The other side of the tree supports a heavy duty russian military fibreglass pole made of ferruled sections bought at a rally, plenty available it seems. This supports an 80m band sized (about 130 feet) off centre fed dipole with 6:1 balun at the feedpoint and two inline chokes, one is 22 feet down, the other at ground level. It matches perfectly on 40m and to get the SWR less than 2.5 I use an ATU on 80m and 40m.

It works well for me - just some ideas which may help you get started. I wish you success with your plans to remote your station. For SOTA a high noise floor is the one thing that makes all the difference from a chasers point of view. Activators mostly run low power with low noise floor. Chasers mostly run higher power with a higher noise floor. It’s surprising what can still be heard and worked though.

73 Phil

9

Thanks for the ideas Phil. Some good ideas there. I had considered using an inverted L.

Still curious about how well an aluminium mast would work if trimmed to a 1/4 wave vertical.

I’m wondering if efficiency would suffer as aluminium is less conductive than copper.

Having said that, it does have a large surface area which may help?

Not sure what the bandwidth would be.

I might have to try this, if only out of curiosity.

10

You wouldn’t notice any difference if aluminium over copper was used on HF for a vertical antenna. The surface area is not so important at all either, keeping it up in the air is! Bandwidth is important on 160m and 80m if you want to use CW and SSB, on other bands a properly matched antenna should be able to cope with the full amateur band with an SWR less than 2:1. Feed it with the best coax that money can buy and I would suggest burying the coax cable in ducting. If a vertical is installed near to property in a residential neighbourhood don’t expect to hear the weaker stations, but they may hear you. Some DXers use ground mounted Spiderpole composite verticals - 18m high, or higher for 80m or 160m - the conductor is just a wire up the side. 12-18 SWG and tuned against two or more ground radials. They are well proven - not even copper or aluminium is needed.

73 Phil

11

Hi James,
Your aluminum mast will no doubt work if you want to use it as a vertical. I home brewed and used for Portable operation some time ago a 5/8 lambda vertical for the 6m band made with copper pipe. The copper pipe radiating element was kept vertically with the help of a non-conductive (plastic) trypod and the ground plane was made with 6 wires of about 5m layed on the ground.
You can see it towards the left side of this picture in my eQSL for that portable operation back in 2009 from Aranjuez-Madrid.
imagen
From that location, I made several QSOs into EU with this antenna and the 5 watts of my FT-817 and I even crossed the Pond making a QSO with V29 in the Caribbean (Antigua & Barbuda Is.)
Using Al instead of Cu will slightly increase the resistance and the RF radiated will be slightly less, but there are so many antennas made of Al in the market performing wonderfully that you can easily figure out how negligible the difference in performance must be.

Regarding the remote station in a property you have away from the urban noise, I highly recommend it. As you know by now, I have recently set up my own remote station and although it’s still an starting one with no antenna switch and antenna rotator yet, I’m enjoying it very much as a RX/TX antenna on the 20m band and also as a RX antenna for the rest of the bands. The way I’m doing this is by having 2 antennas connected in parallel to the radio through an antenna tuner. One antenna is the TH5-DX 5 elements tribander yagi I have on top of a tower about 15m above the ground and the other antenna is an inverted vee broadband folded dipole I have with its appex high about 14m above the ground and hung from the same tower where I have the yagi.
The yagi can’t rotate at the moment and it’s fixed pointing to North-America.
The broadband dipole in that low noise environment lets me copy most or nearly all of the activations in EU when propagation conditions are OK and the yagi let’s me hear those weak signals from NorthAmerica that the dipole can’t copy.
My remote station 2 antennas in parallel are tuned for low SWR on 20m, but I can’t currently transmit on the other bands. So, for the bands other than 20m, I’m receiving in remote with those described antennas and transmitting with my endfed wire in the balcony of my rental appartment in the city and I assure you it’s working very well for me.
I fully support what Ed suggested you in a previous post: if you don’t trust setting up a complete expensive station in your remote low noise location, you can set up a RX remote antenna system (quad or delta loop is a good choice) and keep transmitting from your current noisy location. The SOTA activators will surely copy you.
Good luck and have fun.
73,

Guru

12

Aluminium is an excellent RF radiator. You should have no qualms about using it for a vertical. Commercial trap verticals are all made from aluminium. The difference between aluminium and copper is so small you would never measure or notice it. Look at how the commercial verticals are supported by an elongated U shape bracket with appropriate insulation, copy that idea if possible.

More care is needed with buried radials. That is another topic altogether. if you could keep your radials above ground, say by a metre, you will not need many, 3 or 4 are sufficient. Ideally the radials are in the same plane, otherwise the radiation from the radials does not cancel and you get more signals going up. Buried radials have to overcome ground losses so you need many more. When elevated they create a magnetic and electrostatic field between the radials and the vertical element, with the lossy ground some distance below its influence is far outweighed by the conductive (and resonant, insulated) radials.

Good luck with this antenna, if done well it would outperform many others especially for DX. I got a dxcc with my old 14avq trap vertical, using two radials for each of 40, 20, 15, 10m, as per the manufacturer’s recommendations.

73 Andrew VK1DA

13

Hi,
A remote station is the way to go these days I think.

I’m surprised you would think even for a moment that aluminium tube is inferior to a copper wire for a vertical. If you take a minute and check you will find aluminium has 63% of the conductivity of copper.
You will also find RF flows on the surface of conductors, unlike dc, so the bigger the surface better.
An aluminium tube of 25 mm dia has an rf loss resistance = to 63/10 = 6.3% of a copper wire 2.5 mm in diameter. A bigger tube is even better.
Aluminium tube is favoured as a the material for HF antennas because of its low losses and high strength to weight ratio.
If making a vertical out of several pieces the joins must have low electrical resistance.

All verticals are affected by the ground and quarter-wave verticals work really well with a good radial system and if not screened by buildings or trees.

Any antenna is at least 140 dB better than an ideal one you haven’t yet built and put into service. A wire vertical on a fibre glass pole (if the joins are properly weatherproofed) will last for years but if you have unused aluminium the approach is obvious.

If you have enough aluminium you might consider raising the antenna base and have elevated radials so more of the antenna clears the building. For guying material UV rated plastic cord or light rope is recommended. Check what is used by people who muck about in boats.

Andrew VK1DA has covered the radial issue so no need for me to repeat.

Take out insurance on the remote property so if burgled you are covered.

Good luck with your project.

73
Ron
VK3AFW

1 Like
14

So I know this is a very old topic but I have a follow up question which is somewhat related (hence didn’t really want to start a new topic):-

How would you insulate the mast from the ground?

I was thinking of concreting a 15-20 ft 1.5 inch aluminium tube in the ground to provide a 1/4 wave on 20.

At first glance, concrete would seem like a good insulator. That is until I remembered something that someone told me about grounding my radio tower which is on a large concrete base:-

They said not to put your ground rods through (or even too close to) the concrete because “concrete contains water, the heating from a lightning strike causes the water content to vaporise & blow the concrete apart, basically causing it to explode”.

This makes me wonder if concrete isn’t such a good insulator after all!!!

I note that commercial towers normally use specific insulators at the base. I guess that there must be a reason for this. If they could get away with using the concrete base as an insulator they probably would, if only to bring down the cost.

Failing the concrete idea (which would look a lot neater and could be passed of as a washing line mount when we move house) I guess that a wooden fence post mounted in the ground, with the vertical attached to the wooden post, is the obvious candidate.

Thoughts?