VHF AM Airband Antenna

[prcguy]

Nothing is lossless, but 10 or even 15ft of low loss coax in the phasing harness feeding the furthest element on an UHF version is maybe .5dB, not 3dB.

If you look at major antenna companies that make stacked dipole arrays like DB Products, Sinclair, etc, you will see in their specs that doubling the amount of elements gets you very close to 3dB gain. You can't say the same for stacked colinear types because some of the RF is radiated in the lower elements before it reaches the top.
prcguy

Really? Oh well, perhaps the gain figures Jaybeam, before they became Amphenol used to publish clearly showed that doubling the number of driven elements didn't double the gain - close, but unfair to say they're lossless. In fact most ops the faults we used top swap these out for were caused by degeneration of the harness assemblies, not the antennas themselves. A 6dBd theoretical gain losing just over 1dB in the harness. I realise 1dB is very little - but to say they are lossless is just not right.

 

[rocky28965]

Just winding back to my original post.

If I build a air band vertical/ground plane using the example below.

Build a 2 Meter Vertical Antenna | Mike's Tech Blog | Page 4

Mount it just below the ridgeline of my house.
Will it be more effective than a Nagoya NL-780A on a mag base, if it is mounted on the ridgeline?

I estimate the cost be about the same for either option.

 

[prcguy]

If the Nagoya has some ground plane under it I think it would work slightly better. You should be able to make a 1/4 wave ground plane for almost nothing. All you need is an SO-239 or N type chassis connector, some stiff scrap wire and a big soldering iron.
prcguy

Just winding back to my original post.

If I build a air band vertical/ground plane using the example below.

Build a 2 Meter Vertical Antenna | Mike's Tech Blog | Page 4

Mount it just below the ridgeline of my house.
Will it be more effective than a Nagoya NL-780A on a mag base, if it is mounted on the ridgeline?

I estimate the cost be about the same for either option.

 

[rocky28965]

If the Nagoya has some ground plane under it I think it would work slightly better. You should be able to make a 1/4 wave ground plane for almost nothing. All you need is an SO-239 or N type chassis connector, some stiff scrap wire and a big soldering iron.
prcguy

I was thinking the iron roof would be a good ground plane.
I have a Nagoya UT-106UV perched up on the ridgecap & I'm amazed how well it works.

Cost wise, I would have to buy everything to build a ground plane.
Normally wouldn't, but moved into a smaller place a couple of years ago & had a big cleanout.

 

[majoco]

I was thinking the iron roof would be a good ground plane.

Depends! If your roof is large corrugated galvanised steel sheets, then all you have do is make sure the outer of the coax to the base of the antenna and then to the roof is a good metallic contact - that includes the ridge flashing - you may have to bond the whole lot together (and back to bare metal if painted) with strips of galv. steel preferably soldered but self tapping screws will do.

If your roof is like mine was - coloursteel tiles - I had to make sure all the tiles for a large area were bonded together with sheet metal screws through into the purlins - worked well - unfortunately don't live there any more - great radio site it was too!

 

[rocky28965]

Depends! If your roof is large corrugated galvanised steel sheets, then all you have do is make sure the outer of the coax to the base of the antenna and then to the roof is a good metallic contact - that includes the ridge flashing - you may have to bond the whole lot together (and back to bare metal if painted) with strips of galv. steel preferably soldered but self tapping screws will do.

If your roof is like mine was - coloursteel tiles - I had to make sure all the tiles for a large area were bonded together with sheet metal screws through into the purlins - worked well - unfortunately don't live there any more - great radio site it was too!

Same here coloursteel tiles.
By large area I am thinking about the size of a car.

 

[majoco]

I think I did about a 2metre square with the antenna in the middle of course.

 

[rocky28965]

I have made up a 1/4 wave vertical/ground plane as in the link below

Build a 2 Meter Vertical Antenna - Page 4 of 4 - Mike's Tech Blog

I finished up using 3.2mm alloy welding rods.
At present the lengths are all 600mm
I am not sure what lengths to cut them to as some of the online calculators tend to conflict.

https://m0ukd.com/calculators/quarter-wave-ground-plane-antenna-calculator/

The Ground Plane Antenna Calculator

Which of those is correct?

 

[paulears]

One takes in velocity factor of the cable and the small extra length on the radials - the other doesn't, but I got 571mm on one and 570 one the other - a small difference.

If you enter 118MHz as the centre frequency, you won't have to trim your 600mm lengths at all! As you're not using it for transmit, I'd probably not bother - but the vertical is just over and inch too long - so trim it if you want. In these calculators, they usually say cut long and then trim in ¼" 5mm cuts for lowest VSWR - it's not an exact science, as the diameter, the insulator height at the bottom over the radials and the radial droop all make differences. The basic maths is just 300 divided by the working frequency. This gives the wavelength, so you want a quarter of it. That's very close - and the velocity factor of the cable isn't huge by any means. Many ignore it!

 

[jonwienke]

and the velocity factor of the cable isn't huge by any means. Many ignore it!

Bad idea, ignoring the velocity factor of the coax can throw your calculations off by 20% or more. Most feedlines have a velocity factor between 60% and 80%.

 

[dlwtrunked]

Bad idea, ignoring the velocity factor of the coax can throw your calculations off by 20% or more. Most feedlines have a velocity factor between 60% and 80%.

But they are not talking about coax. The velocity factor of bare metals is near 1..
https://ham.stackexchange.com/questions/1388/effect-of-different-metals-for-antenna-elements

 

[paulears]

Take your point for transmit, of course, but for receive all it does is move the centre frequency down a bit.

 

[jonwienke]

Metal bars are not "cable"...

 

[paulears]

That's a very good point - if you design an antenna, the cable they use you cannot take into account, unless you guess? Probably only vital in phased arrays - 4 stacks and the like.

 

[rocky28965]

Thanks for that guys.
Yes this is only for receive and the feed line is RG58.

 

[majoco]

I have a 'real' aircraft antenna, it's an ELT antenna which I assume is for 121.5MHz and probably was redundant when the UHF and satellite ELT's came along - and it's exactly 600mm from the tip to the mounting flange where it would go through the aircraft skin.

 

[prcguy]

If the ground plane antenna is designed and built properly, then the length of the feedline is irrelevant. The ground radials will do a good job of decoupling the feedline from the antenna.
prcguy

That's a very good point - if you design an antenna, the cable they use you cannot take into account, unless you guess? Probably only vital in phased arrays - 4 stacks and the like.

 

[rocky28965]

Thanks Martin, sounds like I'm pretty close.
Just about to try this thing.
Going to sit it on 3 meters of condute and stick it down the hole in our patio table.

 

[rocky28965]

Well it seemed to work ok, was getting NZQN tower, both approach & delivery, also Oamarama glider
base.
So it's slightly better than my vertical dipole.
So instead of placing it on the back vent, it now resides on the sky dish pole instead of the dipole. .
It looks a bit more elegant sitting up there too.