First antenna build problems....

[FNH]

Hi guys,

Firstly please excuse any errors in my descriptions, I am pretty new to this.

I have just made my first attempt at an antenna to use for a base station (pic 1). It is a quarter wave ground plane and I am intially trying to tune it in for the Kiwi AM band which is 26.330 - 26.770 and I am struggling to get the SWR down to a reasonable levels. The best I can get is high 3's at either end with the middle being about 0.2 less. So Ch1 = 3.8, Ch19 = 3.6 & Ch40 = 3.8.

The construction is a steel angle bracket mounted on the iron roof vertically with 4 braces welded to it (pic 2). To this is clamped a steel riser tube with a flange welded on top and 4 guy ropes for support. This has a plastic mount for the SO239 near the top, with the coax cable going into the tube and upwards. On to this is bolted an alloy base (pic 3) for the radial's, which are alloy tubes pushed into holes and retained from underneath with grub screws. The outer shield of the coax is attached to the inside of alloy base. The alloy base has a large hole running through it and in the hole is clamped a plastic cup which in turn has the vertical antenna clamped into it. The cup insulates the vertical antenna from the alloy base. The inner insulated core of the coax passes through a hole in the bottom of the cup and attaches to the bottom of the vertical antenna. The vertical antenna is made from two alloy tubes (pic 4), with one sliding inside the other for tuning and the upper tube is closed at the top with a stainless spike.

The vertical antenna is 2.685m long.
The radials are 2.685m tip of one to tip of opposite one (at 180 degrees).
The coax is 7.5m long from the SO239 to the transiever and has no kinks or sharp bends in it.

I have checked continuity with my meter and all seems good. I have added an extra earth cable (since the pictures were taken) to bond the steel riser tube and the steel angle bracket to the iron roof.

I have checked the SWR with a twin guage SWR meter and a single guage type that you switch from Set to Reflected and both read about the same. Adjusting the vertical antenna's length does change the readings. Though the lowest reading never gets below high threes.

The only thing I am wondering is whether I should have insulated the flange on the steel riser from the alloy radial mount? At the moment they are not insulated from one another.

The reason for the extra two radials at 45 degrees, is because there is a hill behind us and dead ground. All the action is to the front and sides, so I wondered if the extra radials would help weight the signal to the front & sides more. I may have got this wrong though?

I just don't know whether I have overlooked something small, got my numbers wrong or made a huge cluster-F! I am really hoping it is the former of the three.

So if anyone has any suggestions, thoughts or observations I would be most gratefull.

TIA FNH


Pic 1

Pic 2

Pic 3

Pic 4

 

[222DBFL]

Pic 3 looks like a bad connector, I see shielding, and is it a crimp on style connector, if so, cut it off and add new connectors that are soldered on both ends of your coax. And also is that is 50 ohm coax? Just asking.

 

[222DBFL]

And also can you remove the wraps of coax around the pole and let the coax come straight off the pole with no loops in it.

 

[FNH]

Pic 3 looks like a bad connector, I see shielding, and is it a crimp on style connector, if so, cut it off and add new connectors that are soldered on both ends of your coax. And also is that is 50 ohm coax? Just asking.

Thanks for the quick response.

That cable was the only thing I didn't do! I got a shop in town to make it up for me. Yes it is a crimp type, I cannot check the one in the picture (dark & raining), but the one on the other end inside looks neat and the crimped collar is slotted, which may account for it looking like the shielding is showing in the picture, but maybe not. I will check it out though. Yes it is Pacific Aerials RG58 A/U, which is 50 ohm.

And also can you remove the wraps of coax around the pole and let the coax come straight off the pole with no loops in it.

After initially trying it out and after further reading, I wrapped the coax round the tube to see if it made any difference. It didn't!

 

[Robb]

First blush, the antenna radiator is 105.7 inches long, which means that it is too short for the 26mhz range. Would tune closer to 28mhz at that length.

A 1/4 wave antenna is a very simple antenna to make; the very first antenna I've ever built. I used ~108 inches for the center conductor/vertical, and then I made four radial at ~105 inches attached to the coax shield, and then set the radials slope at ~35 degrees. All made out of 10ga wire; it worked fine the first time off using just scrap materials. That was thirty years ago - BTW.

It is really hard to figure out just went wrong, other than you not using four 1/4w long radials instead of six shorter ones. Having a downward slope on the radials will also affect the SWR too, and your antenna's radials don't have any slope at all. But what concerns me the most is your feedpoint PL connector and how you have wired it to the vertical and the radials. Are you sure there is no conductivity between the vertical and the radials?

 

[wavrider]

First blush, the antenna radiator is 105.7 inches long, which means that it is too short for the 26mhz range. Would tune closer to 28mhz at that length.

A 1/4 wave antenna is a very simple antenna to make; the very first antenna I've ever built. I used ~108 inches for the center conductor/vertical, and then I made four radial at ~105 inches attached to the coax shield, and then set the radials slope at ~35 degrees. All made out of 10ga wire; it worked fine the first time off using just scrap materials.

It is really hard to figure out just went wrong, other than you not using four 1/4w long radials instead of six shorter ones. Having a downward slope on the radials will also affect the SWR too, and your antenna's radials don't have any slope at all. But what concerns me the most is your feedpoint PL connector and how you have wired it to the vertical and the radials. Are you sure there is no conductivity between the vertical and the radials?

ROBB pointed out some major issues, also the antenna being so close to the metal roof will have some effect on the VSWR, you may also want to try an ugly choke at the feed point.

BUILD AN AIR WOUND 1:1 CHOKE BALUN FOR HF - THE UGLY BALUN!

I think for 26 mhz probably around 5 turns of coax around a 4 1/2" diameter tube will work. I am sure some others will correct me if this dimension is incorrect.


Having the ground radials at a 45 degree bend is a MUST to get the impedance down to 50 ohms.

Enjoy and have fun.

 

[FNH]

First blush, the antenna radiator is 105.7 inches long, which means that it is too short for the 26mhz range. Would tune closer to 28mhz at that length.

A 1/4 wave antenna is a very simple antenna to make; the very first antenna I've ever built. I used ~108 inches for the center conductor/vertical, and then I made four radial at ~105 inches attached to the coax shield, and then set the radials slope at ~35 degrees. All made out of 10ga wire; it worked fine the first time off using just scrap materials.

It is really hard to figure out just went wrong, other than you not using four 1/4w long radials instead of six shorter ones. Having a downward slope on the radials will also affect the SWR too, and your antenna's radials don't have any slope at all. But what concerns me the most is your feedpoint PL connector and how you have wired it to the vertical and the radials. Are you sure there is no conductivity between the vertical and the radials?

Okay thanks, so I have some where along the lines, got the numbers wrong. I did trim the radial tubes down a bit too and made them 2.685. Whoops.
Here's a rough section of the internal connections.

ROBB pointed out some major issues, also the antenna being so close to the metal roof will have some effect on the VSWR, you may also want to try an ugly choke at the feed point.

BUILD AN AIR WOUND 1:1 CHOKE BALUN FOR HF - THE UGLY BALUN!

I think for 26 mhz probably around 5 turns of coax around a 4 1/2" diameter tube will work. I am sure some others will correct me if this dimension is incorrect.


Having the ground radials at a 45 degree bend is a MUST to get the impedance down to 50 ohms.

Enjoy and have fun.

Okay thanks. It was a version of that article I read, that prompted me to try winding the coax around the pole.

So next step is to extend the radials back to what they should be and bend them down 35-45 degrees.

You know this can only get better...

EDIT>>>>>>

I have just double checked using 26.550 in a couple of online calculators (http://www.csgnetwork.com/antennagpcalc.html & http://www.qsl.net/kb5wck/antenna.html)
They both come up with my original figure of 2.685m (8.81') for the centre and radials, so I might just bend them down first and see where that gets me.

 

[W5LZ]

Using those calculators certainly makes for easier construction but keep in mind that they are never going to be exact about lengths. There's always a little 'trimming' required, if for no other reason than because of the antenna's surroundings, nearness to stuff. I don't think it will be required, but it's also a bit easier to extend an antenna's length before cutting things, make sure you are going in the 'right' direction with that length adjustment, you know?
Just from the figures you provided, adjusting the 'tilt' of the radials isn't going to do it by it's self. Shortening the vertical element would be the first thing, and once things get close, then do the bending of those radials.
As for the choke on the support pole, you may not need it at all, won't know till you try without it. If it isn't needed, don't put it back.
I think it's a very nice looking antenna. The feed line attachment block is certianlly a bit different than what's normally seen, but it should work just fine. And the design of the radial hub should be fine too. The proper connections are made 'inside'.
I would recommend that the 'sharp' point at the top be changed slightly, put a more 'blunt' or rounded point on it instead. Sharp things tend to discharge energy in 'sparks', sort of (corona discharge). Rounded, or a ball on the tip tends to minimize that corona discharge. That discharging may look impressive but it does nothing but add noise to the system. Unless you plan to run lots of power that 'ball' doesn't have to be large at all.
If the PL-259 crimp-on connector works for you then why not use it. I don't have much faith in them, would rather use a soldered connection, but that's just personal preference. If done correctly they do just fine.
Mounting the thing close to a metal roof is going to change it's electrical behavior in some ways. Getting it higher tends to minimize the reaction between antenna and metal roof so it isn't a bad idea. Or, mount it closer to that metal roof and use the roof as the 'radials'. That works too.
The number of radials used isn't critical at all. You need at least one, and more isn't going to hurt. When you get tired of adding radials, quit! It also means that the 'tilt' of those radials may not be the same as with less radials. That's just a part of the tuning process (so is the amount of 'tilt').
The amount of directivity of a vertical antenna can be 'adjusted' slightly by where you place those radials, but don't expect that directivity to be 'large', it won't be. It -may- help in your situation with that hill, but it won't be a "daylight/dark" kind of thingy.
So, try what you think may help and see what happens. It ought'a be interesting!
- 'Doc

 

[Road Squawker]

overall, nice construction.

is it spray painted with black paint?,... or perhaps flame sprayed with powder coating?

 

[FNH]

Using those calculators certainly makes for easier construction but keep in mind that they are never going to be exact about lengths. There's always a little 'trimming' required, if for no other reason than because of the antenna's surroundings, nearness to stuff. I don't think it will be required, but it's also a bit easier to extend an antenna's length before cutting things, make sure you are going in the 'right' direction with that length adjustment, you know?
Just from the figures you provided, adjusting the 'tilt' of the radials isn't going to do it by it's self. Shortening the vertical element would be the first thing, and once things get close, then do the bending of those radials.
As for the choke on the support pole, you may not need it at all, won't know till you try without it. If it isn't needed, don't put it back.
I think it's a very nice looking antenna. The feed line attachment block is certianlly a bit different than what's normally seen, but it should work just fine. And the design of the radial hub should be fine too. The proper connections are made 'inside'.
I would recommend that the 'sharp' point at the top be changed slightly, put a more 'blunt' or rounded point on it instead. Sharp things tend to discharge energy in 'sparks', sort of (corona discharge). Rounded, or a ball on the tip tends to minimize that corona discharge. That discharging may look impressive but it does nothing but add noise to the system. Unless you plan to run lots of power that 'ball' doesn't have to be large at all.
If the PL-259 crimp-on connector works for you then why not use it. I don't have much faith in them, would rather use a soldered connection, but that's just personal preference. If done correctly they do just fine.
Mounting the thing close to a metal roof is going to change it's electrical behavior in some ways. Getting it higher tends to minimize the reaction between antenna and metal roof so it isn't a bad idea. Or, mount it closer to that metal roof and use the roof as the 'radials'. That works too.
The number of radials used isn't critical at all. You need at least one, and more isn't going to hurt. When you get tired of adding radials, quit! It also means that the 'tilt' of those radials may not be the same as with less radials. That's just a part of the tuning process (so is the amount of 'tilt').
The amount of directivity of a vertical antenna can be 'adjusted' slightly by where you place those radials, but don't expect that directivity to be 'large', it won't be. It -may- help in your situation with that hill, but it won't be a "daylight/dark" kind of thingy.
So, try what you think may help and see what happens. It ought'a be interesting!
- 'Doc

Thanks. Top tube of the vertical is 0.5" diameter and I started out with a tip in the end that was tapering down to a 0.125" radius. After first testing I wondered if this was part of the problem and made the tip taper down to a radius of about 0.0625". I'll make up another at 0.25" and see what that does. Not having any plugs or coax cable here, I thought getting a shop to make that up would be the best option and so I just have used what I was given. I will source some solder plugs and remake the ends with those.

overall, nice construction.

is it spray painted with black paint?,... or perhaps flame sprayed with powder coating?

All the steel support bits are zinc plated and then painted black and all the alloy antenna bits are black anodised. (some products I make are finished this way, so getting the antenna parts done was not an issue). All the fixings on the antenna are stainless and where the coax attaches inside I have made brass lugs and soldered the cable to them. I checked continuity as I assembled it to make sure I was getting good connections etc. Apart from the coax cable and a few SS grub screws, it hasn't cost me anything to build, just my time.

Thanks for all the advice you guys have given on this.

FNH

EDIT>>

Just a comment on the cross section sketch I posted. I have noticed I have drawn it slightly wrong last night. The hole in the steel flange is slightly smaller, than the hole in the alloy base that sits above it. Also the plastic insulator cup is longer and extends down and sits on the flange. This shouldn't make any difference to the problem I am having, but just shows the parts fit together better and are better supported.

 

[wavrider]

Okay thanks, so I have some where along the lines, got the numbers wrong. I did trim the radial tubes down a bit too and made them 2.685. Whoops.
Here's a rough section of the internal connections.

Okay thanks. It was a version of that article I read, that prompted me to try winding the coax around the pole.

So next step is to extend the radials back to what they should be and bend them down 35-45 degrees.

You know this can only get better...

EDIT>>>>>>

I have just double checked using 26.550 in a couple of online calculators (Amateur Quarter Wave Ground Plane Antenna Calculator & Antenna Calculations)
They both come up with my original figure of 2.685m (8.81') for the centre and radials, so I might just bend them down first and see where that gets me.

234/frequency.

234/26.550=8.81 or roughly 105" or 2.667 meters

That is the length of your vertical and also your ground radials.

three or four radials are all you need to make this ground plane efficient.

 

[vkrules]

Hi.The only thing I can see is that the ground radials are half the length they should be.Try extending at least 3 of them and see what happens.If you don't have any more tubing use wire with a insulator on the end and tie it down to the roof, (like guy wires) Can you test swr a bit higher and lower in frequency to see if there is a dip outside of the band your using.

 

[vkrules]

Just had a thought, check continuity between tube sections. anodizing isn't the best conductor

 

[W5LZ]

There are different types of anodizing. Some are insulators and some conduct. No idea what to call either so I guess try it and see.
- 'Doc

 

[Robb]

Hi.The only thing I can see is that the ground radials are half the length they should be.Try extending at least 3 of them and see what happens.If you don't have any more tubing use wire with a insulator on the end and tie it down to the roof, (like guy wires) Can you test swr a bit higher and lower in frequency to see if there is a dip outside of the band your using.

Yep; there should be three/four radials @ ~105 inches each at a ~40 degree downslope.