HF Verticals... How close is too close?

[VK3LU]

How close can metal objects be before they are deemed too close to an HF vertical?
I can't find any info on the internet so far. Maybe I'm using the wrong words!
I am looking at the feasability of installing a 43 ft vert. and the only location I can put it is 5 metres from the metal clad wall of the back of the house ( which is in the europe direction ) and 6 metres from the metal panel boundary fence.
Has anyone had any experience in this regard?

73 Nev

[VK5TM]

If you can find a copy of NMEA 0400, there is information in there (it applies to boats and ships, but seeing as they are mostly metal, the info should be comparative to your situation).

Any chance of raising the vertical above the metalwork? Would probably mean using elevated radials though.

Otherwise, it's going to be a case of erecting it temporarily in various spots to ascertain the best operating point within that area.

[VK3LU]

Hi Terry,
As I stated in the original post, there is only one spot I can put it. It's there or nowhere.
Elevated radials are out of the question. 
I will have a look for NMEA 0400 and see what it says.

73
Nev

[VK4ADC]

Why not put it about 1 metre ( + / - whatever suits) from the shed and then bond the coax outer to the shed to use as the virtual ground plane ?  It will definitely need a ground plane so why not make the shed fulfill that task. 

If you can elevate the base of the vertical a little bit (ideally the shed height at the side where the vertical mounts) then that is even better. The bonding will be more direct. None of the neighbours will see any difference between 43 feet and 50 feet high

Ditto the metal fencing. Put the vertical near the fence and use that as the ground plane. ( I did that trick years ago with a multiband trap vertical when I had a long chainwire fence with top piping joining all of the posts - worked well even with those untuned "radials" going in just two directions. The trap vertical was clamped/attached to the top of one of the galv fence posts.)

If the shed and fence "meet" then put the vertical near the junction and use all three bonded together.  

The outer braid from RG213 coax  is great for that bonding - cut a suitable length then grip a bit of the exposed inner in a vice and pull it out of the braid + protective sheath.  Trim to length, expose about 50mm of braid and flatten it with your fingers, pierce the flat bit with a philips screwdriver to make a neat hole then tin (solder) the exposed braid. Easy to put a metal screw or bolt up to 6mm through to do the metal bonding bit.  Seal up the ends of the coax outer to stop moisture getting in and causing corrosion of the copper braid - non-acidic silicone is great, in black is better than clear for outside use..

PS That same trap vertical spent quite some time (about 4-5 years) in the centre of another galv sheeting roof - but I did spend some time making sure that the galv sheets were actually joined together electrically. I worked all over the world using that arrangement.   It now resides at my grandson's home (F-call) with it attached to the top of yet another chainwire fence section, 40 years after it's first placement on one.....

[VK3LU]

Hi Terry,
I checked out NMEA 0400 and found I would have to cough up $638 US for a copy! So much for that.

Doug,
I doubt if it would be feasible to mount the vertical on either the rear of the house (not a shed) or the boundary fence.
The rear of the house is about 3m high and the boundary fence is one of those aluminium panel jobs that are prone to blow over in very high winds.
What I have considered is connecting the above into the ground radial system and see if that works.
But I am still seeking an answer to my question.

Nev

[VK4ADC]

Nev,

Re-read your post and apologise for reading it as shed instead of metal-clad house walls.  My error.


Keep in mind that metal cladding - as well as the colourbond-style fencing - doesn't actually have to have a full electrical contact (ohmic) between each piece.  The metal overlaps, even though insulated by paint, still have fairly high sheet-to-sheet capacitance if the overlapped areas are large enough. The dielectric spacing is quite small ( 2 x paint thickness in most cases) and the dielectric constant is likely to be high.  That "capacitance effect" is likely to provide a low capacitive reactance at HF.

As for distance, I would suggest 1 - 2 metres would be a reasonable enough separation, and as Terry suggested : Try it.  Your physical environment is the determining factor so what it is, it is. Experiment.

I use my 7M squid pole (wire up the middle) about 500mm from the body of the caravan, the base is a pipe u-bolted onto the drawbar, and a similar pipe mount on the rear bumper about 300mm away from the body there. Both mounts provide similar results on HF - as attested by the reports on FT8 while wandering the countryside. Proximity doesn't seem to be an issue.

I used to live in suburbia - for some 38 years in houses we owned and with all of the limitations of suburban blocks and nearby neighbours - but about 6 years ago, we moved to a semi-rural environment. I now have a 4350 SQM block, enough space for 2 tilt-over towers complete with yagis on all bands from 20M to 23cm plus full size multi-band wire dipoles for 160 / 80 / 40 / 30M plus miscellaneous verticals for 6, 2 and 70cm.  It was a big move for us but it gives me the antenna opportunities I never had access to in suburbia. The neighbours are further away and don't care what antennas - or how many - I put up (I've asked!). And less HF band noise too. Best move ever.

PS Just did a quick count : 17 antennas up in the air right now, more in the shed for FDs, caravanning, portable operations, then more 6 / 2 /70 cm yagis stored outside the shed wall. I just don't count the other things like gridpack reflectors stored outside the shed too.