5/8 Wave vertical antenna question

[3metrejim]

The two graphs at the top of this linked page will be helpful for anyone designing their own antenna.

https://www.qsl.net/kd2bd/impedance_matching.html

The graphs seem to bear out my thoughts on how my antenna was matched: anything over about 1/2 wave shows a capacitance with a resistance of over 50 ohms. Using a longer than 1/2 wave element reduces the feed point impedance compared to a half wave keeping the RF voltages lower.

On the capacitor I used for tuning: Most commercial antennas are for a fixed, narrow, frequency range. To cover a wider band you need a tunable element, just altering the antenna length won't be sufficient - the reactive value of the antenna will stay the same, but that of the coil at the base will change with frequency, so you need a variable inductor (hairpin) or a variable capacitor in my case (also the NRG 1/2 wave). For a fixed frequency, you can wind the matching coil with enough accuracy to get a low SWR, capacitance comes from the coil itself due to interwinding capacitance and the distributed capacitance of the antenna element. On a cheap silver rod type CB antenna there is also capacitance between the antenna rod and it's mounting bracket. Any two separated pieces of metal act as a small capacitor.

I tried antennas without a groundplane and they all seemed to suffer from detuning due to mounting, or movement of the co-ax cable. With a groundplane there was no difference mounting on a insulated or conductive support. Also the level of RF below the antenna was much reduced, I used a wavemeter to verify that (and my audio equipment stopped buzzing too)

[Albert H]

As Jim (above) says - the presence of a ground plane or a counterpoise makes the aerial behaviour much more predictable. Stephen's ½-wave vertical was fine since there was a counterpoise, but the performance of the aerial could be significantly improved by extending the (earthed) mounting post. I also found that the radiation angle of the ½-wave was great for low sites and gave a nicely flattened radiation shape with little energy wasted either upwards or into the ground.

The ⅝-wave gave a more sensible match, but a poorer radiation angle. Best of all was the ⅞-wave, which, with ground planes, was by far the best vertical I ever tried. The signal in the block was tiny and there were never any RF feedback hum problems, and at ground level beneath the site, it seemed to be coming from somewhere else! When you got ½-mile away from the block, it was HUGE! I used one of those from the "Battleship" block near Forest Hill with just 60 Watts going into it, and it gave a cracking signal in Epping!

I have a little station about 2km along the road from my house, and they've just changed from a twin stack dipole to a ⅞-wave with ground planes, and their field strength here - with the same rig - has more than doubled!

[nrgkits.nz]

As Jim (above) says - the presence of a ground plane or a counterpoise makes the aerial behaviour much more predictable. Stephen's ½-wave vertical was fine since there was a counterpoise, but the performance of the aerial could be significantly improved by extending the (earthed) mounting post. I also found that the radiation angle of the ½-wave was great for low sites and gave a nicely flattened radiation shape with little energy wasted either upwards or into the ground.

The ⅝-wave gave a more sensible match, but a poorer radiation angle. Best of all was the ⅞-wave, which, with ground planes, was by far the best vertical I ever tried. The signal in the block was tiny and there were never any RF feedback hum problems, and at ground level beneath the site, it seemed to be coming from somewhere else! When you got ½-mile away from the block, it was HUGE! I used one of those from the "Battleship" block near Forest Hill with just 60 Watts going into it, and it gave a cracking signal in Epping!

I have a little station about 2km along the road from my house, and they've just changed from a twin stack dipole to a ⅞-wave with ground planes, and their field strength here - with the same rig - has more than doubled!

Very interesting to know, so the 7/8 gave about 3dB more gain over the twin stack? I might give one of these a go and see how it compares with my jpole.

[Electronically]

nrgkits I disagree with the extended mounting pole on Stevens 1/2 antenna because once it's clamped to metal pole for support it wouldn't need extended its already there .

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[Electronically]

The two graphs at the top of this linked page will be helpful for anyone designing their own antenna.

https://www.qsl.net/kd2bd/impedance_matching.html

The graphs seem to bear out my thoughts on how my antenna was matched: anything over about 1/2 wave shows a capacitance with a resistance of over 50 ohms. Using a longer than 1/2 wave element reduces the feed point impedance compared to a half wave keeping the RF voltages lower.

On the capacitor I used for tuning: Most commercial antennas are for a fixed, narrow, frequency range. To cover a wider band you need a tunable element, just altering the antenna length won't be sufficient - the reactive value of the antenna will stay the same, but that of the coil at the base will change with frequency, so you need a variable inductor (hairpin) or a variable capacitor in my case (also the NRG 1/2 wave). For a fixed frequency, you can wind the matching coil with enough accuracy to get a low SWR, capacitance comes from the coil itself due to interwinding capacitance and the distributed capacitance of the antenna element. On a cheap silver rod type CB antenna there is also capacitance between the antenna rod and it's mounting bracket. Any two separated pieces of metal act as a small capacitor.

I tried antennas without a groundplane and they all seemed to suffer from detuning due to mounting, or movement of the co-ax cable. With a groundplane there was no difference mounting on a insulated or conductive support. Also the level of RF below the antenna was much reduced, I used a wavemeter to verify that (and my audio equipment stopped buzzing too)

3meterjim that was a very intersting read .yup I agree it is narrow on frequency which is good .cause the antenna you want is to make it go 2mhz each side .And I'll certainly be attempting that 1/2 1/4 coil idea .

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[Electronically]

correct me right or wrong here on Stevens 1/2 vertical the rg213 coax that was attached .Was this a 1/4 stub ? .Just by looking at it to be short length looks like a stub .Because I remember once phoning the guy I said could you send coax with this he sent rg58 coax which attaches to the rg213 .

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[nrgkits.nz]

correct me right or wrong here on Stevens 1/2 vertical the rg213 coax that was attached .Was this a 1/4 stub ? .Just by looking at it to be short length looks like a stub .Because I remember once phoning the guy I said could you send coax with this he sent rg58 coax which attaches to the rg213 .

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The RG213 is just a tail, not a tuning stub. Tuning is done using an LC network at the base of the radiator.

[Electronically]

Always do remember guys .If your antenna impedance is 52 ohms and your coax is 52 ohms that's a 1.0 swr .so basically just divide whatever your impedance is .so 52 divide by 52 =1

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[Electronically]

correct me right or wrong here on Stevens 1/2 vertical the rg213 coax that was attached .Was this a 1/4 stub ? .Just by looking at it to be short length looks like a stub .Because I remember once phoning the guy I said could you send coax with this he sent rg58 coax which attaches to the rg213 .

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The RG213 is just a tail, not a tuning stub. Tuning is done using an LC network at the base of the radiator.

I know the tuning lc is done on radiator .But was checking why there was rg213 coax attached at a specific length so wondered if it's a stub .

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[Albert H]

No. The RG213 length was just convenient for the support pole that Stephen supplied!

As an aside - there's a version of the NRG ½-wave that we used to make that has the coax coming down inside the support pole. This helped prevent coax radiation and in one instance allowed us to conceal a PA inside a supporting scaffold pole, using the pole as the heatsink too! There were two leads coming out of the bottom of the scaffold pole - RG58 which we fed 1 Watt up, and a red +30V lead that fed the PA, which was a 2N3375 / 587BLY job that gave around 90 Watts out. With the antenna gain and the excellent radiation shape, this combination worked really well!

[Electronically]

Thanks Albert. The one you described the coax inside the pole .was this never released for sale .Just can't think of the one your describing there

No. The RG213 length was just convenient for the support pole that Stephen supplied!

As an aside - there's a version of the NRG ½-wave that we used to make that has the coax coming down inside the support pole. This helped prevent coax radiation and in one instance allowed us to conceal a PA inside a supporting scaffold pole, using the pole as the heatsink too! There were two leads coming out of the bottom of the scaffold pole - RG58 which we fed 1 Watt up, and a red +30V lead that fed the PA, which was a 2N3375 / 587BLY job that gave around 90 Watts out. With the antenna gain and the excellent radiation shape, this combination worked really well!

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[Albert H]

Stephen and I made a couple with the coax inside the pole. They were also made with (selected) fixed capacitors and carefully bent coils! The enclosure for the first one we made was an empty Squeezy bottle, and the one I've still got on a site in Romania has a Coke bottle housing the coils and capacitor!

When that aerial worked properly, it worked really well.

[nrgkits.nz]

image.jpeg

Here's mine, I cut the bottom off a coke bottle and stuck it upside down over the coil and capacitor.

[Electronically]

image.jpeg

Here's mine, I cut the bottom off a coke bottle and stuck it upside down over the coil and capacitor.

Thanks nrg but could take a lose up if it just it's a bit in distance

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[3metrejim]

Nice set up, nrgkits.nz. 300mW should go some distance from that location. What linking system are you using? Looks to be some sort of microwave link set up.

[nrgkits.nz]

Nice set up, nrgkits.nz. 300mW should go some distance from that location. What linking system are you using? Looks to be some sort of microwave link set up.

They are ubiquiti PowerBeams - used to link with my remote site up on the hill in the far distance (you can see a tower in the distance up on that hill) my site is 900m to the left on the same ridge and gets all of Auckland from up there. The nrg half wave in this photo is just a test antenna which I test transmitters on. The other dish's are linking to other locations where I have customers paying for fast broadband - as we have 1gbit fibre at the house in this photo.

[Electronically]

image.jpeg

Here's mine, I cut the bottom off a coke bottle and stuck it upside down over the coil and capacitor.

Thanks nrg but could take a lose up if it just it's a bit in distance

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Sorry meant could you take a close up of antenna. it's just a bit in the distance .

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