Other SOTA sites: SOTAwatch | SOTA Home | Database | Summits | Video | Photos | Shop | Mapping | FAQs | Facebook | Contact SOTA

HF Linked dipole

The build of the HF linked dipole is under way. Aiming for 20m and 40m band.
So far, 20m is done and by the power of NanoVNA and MFJ HF antenna analyser the SWR figures compare quite closley
Started with 20m construction

First contact, Italy on 20m.

Came back to the antenna next day and nothing at all. No reception. ATU would not tune up. Discovered a faulty centre pin on the BNC connector. Resolved this by replacing the BNC plug.

Next is 40m, but at the moment, SWR is 1.8.- 2.0 a little high. Need to check the connections and try again.

Cheers
John
m0vaz

5 Likes

Centre pins on bnc plugs can trap you sometimes. I use rg174 feeders with bnc be very careful when you are putting up the dipole you don’t step on the coax and give it a tug.
My centre pin on the antenna end got pulled out a bit not really noticeable to look at but enough not to make a good connection, that was caused by me having some of the coax under my feet as I slid the telescoping pole up. Good test for a dipole you know to be resonant should not need a tuner really but I always “nip” it up anyhow with my KX3 and if the antenna is good the rig will only take a very short tune but if its grinding away to find a good tune something may be wrong to upset the tuner.
Hope the next stage goes ok. Cut 2 x 9.3 m of wire extra and clip them onto the ends of your 40/20 dipole to get you on 80m. In a pinch my KX3 will tune my dipole on 30m and 17m if you have one side as a 40m leg and the other side unclipped as a 20m leg like an OCF dipole I got my first S2S with USA NJ7V on 17m that way as I did not have a proper 17m antenna with me. Off course your 40m dipole will tune on 15m too in a pinch also, might be some feed line losses but could still get you a score on those bands.
Regards
Ian vk5cz …

2 Likes

Hi Ian, great hints and details of your experiences with coax and bnc connectors, Thanks. :slightly_smiling_face:

73 de Geoff vk3sq.

Hi John,

Its unlikely you will get as good match on 40 m as you have on 20 m unless you double the height agl. The RSGB Handbook used to have a nice graph of feed resistance vs height.

Google tells me that a 7 MHz dipole at 20 m agl will have a feed resistance of 71 ohms resistive and an SWR of 1.4:1 for 50 ohm coax. Lowering this dipole to 8 m agl oddly gives the same resistance but adds 56 ohms of inductive reactance, spoiling the SWR. (2.6:1) Dropping it further to 4 m pulls the resistance down to 23 ohms with 39 ohms of inductive reactance (3.6:1). Adjusting the length for best SWR at your intended height will yield the best results without a tuner or other matching. Which might be where you are now.

SOTABeams uses thin coax and thin wire that have some inbuilt losses and hence appear as better matched at any height. Ground resistance is of course quite variable.

In my more recent SOTA activations I always had an ATU and usually a 5 MHz doublet fed with 300 ohm ribbon. I figured if I needed an ATU I might as well give up on resonant half waves and go for the smallest antenna that will work well on 80 m. I can QSY to any frequency (3 - 55 MHz) in 15 seconds. Lazy.

Tony VK3CAT has always used a double 5/8 for 20 m with 300 ohm feed to his KX2 to cover the same range. Its taken me a decade to come to a similar arrangement,

Now I’m using something else but its not for SOTA.

73
Ron
VK3AFW

1 Like

Thanks for the tips chaps. The BNC centre pin issue was at the radio end, I noticed if I gave the coax a nudge the signal reception would jump up.

I am currently working at a 5m/6m antenna height as that’s all the height I have at the mo for a centre support…and also a small garden. I had to move the mast around the garden and one leg of the wire into the drive between buildings.

I think a trip into a big field armed with a NanoVNA to see how the antenna performs.
But so far quite happy with a first HF antenna build.
Cheers
John
m0vaz

Hi John,
There are 75 Ohm and 50 Ohm versions of the BNC plug and I believe they have different diameter centre pins. Could this have been part of the problem?

As for the correct element length based on the height on a linked dipole, the calculator here: Extras - various tools for SOTA purposes - sotamaps.org takes all of that into account I believe.

73 Ed.

2 Likes

I did check spec of the ones I found a drawer at work (office getting closed down/most of the old contents binned)
https://uk.rs-online.com/web/p/coaxial-connectors/0404985/ 50Ohm.
Also found a whole load of BNC cable boots in various colours :slight_smile:

Ed,
You are right.
A lot of people get into trouble mixing 75 ohm and 50 ohm connectors. The 50 ohm males wreck the 75 ohm females and the 75 ohm males rattle around in the 50 ohm female sockets. Never designed to work that way.

Of course there may be other reasons why rattling the connector makes and breaks the connection.

BNC
Bloody Nasty Connectors.

N connectors are Nice. But again keep to the same impedance.

PL 259
Proper Large connector 2 get 59 reports.

73
Ron
VK3AFW

1 Like

Hi Ron,

Sorry, but I made only very little bad but a lot of good experiences when using BNC for more than 40 years for the job and for the hobby - except crimped BNC. After some experiences I don’t use crimped BNC for the last 20 (or so) years. And please use only qualitative ones and follow the assembly instructions!

Of course, take especially care when using thin coax in the field, it’s not a rope.

73 Ludwig

The beauty of a 5m pole (and possibly) 6m pole is that you can reach the links usually from ground level without dropping the aerial pole. Just about to build another link dipole myself, for 40/30/20m lighter and thinner than before with 22 SWG silicone wire in blue and fed with RG-174 coax. The whole lot should fit on a single winder. I hard wire the top - no connector and no balun, KISS method and saves weight and possible breakage point. VNA not needed, but I do have a Rig Expert AA30 analyser, got it Freidrichshafen some years ago.

73 Phil

73 Phil

2 Likes

It’s understandable. ConsiderRG58 cables and BNC connectors that use the clamp method. The braid is folded back over a metal fitting and then it is held in place by the rubber gland that gets compressed as you screw up the end of connector. The gland grips firmly around the outer PVC sheath and that takes the majority of the cable strain leaving the shield and centre dielectric and conductor to carry the signal not the physical load.

In a crimp version, the braid goes over the tail of the connector body and then the crimp fitting is compressed to hold the braid firmly in place. At this point there is no strain relief. Normally you fit some heatshrink, preferably with internal glue, to compress and adhere to the crimp fitting and PVC outer. This takes a lot of strain off the braid. The flexible plastic strain relief taills help by limiting the amount you can flex the cable at the crimp. But ultimately you need something that is well attached to the PVC outer and the crimp to take the physical load.

I think a crimped connector is always going to be more susceptible to physical shocks and loading than clamp connector. I only just started using RG174 BNC crimp connectors instead of using clamp connectors for RG58 and “McGuyvering” some kind of strain relief from heatshrink and hot melt PVA glue. The crimped connectors are much lighter and don’t look like Stevie Wonder assembled them.

As the man said…“you pays your money and you takes your choice.” and crimped connectors do look better.

1 Like

DD5LP - I used Sotamaps for the measurements/lengths of the wire. :slight_smile:

Hi Ludwig,

Well I’ve had a number of screw type BNC connectors fail. Even MIL spec ones . No crimped ones have failed.

Unfortunately my rheumatism prevents me from being able to always complete a crimp properly.
Nevertheless I’m not going back to the rubbery BNC.

The PL259 is prefered here as I can still assemble them and there’s nothing like a soldered joint for good electrical contact.

As someone else mentioned, hard wiring the coax to the antenna or matching transformer removes the need for a connector.

Yes I know, very 1940ish.

73
Ron
VK3AFW

1 Like

Hi Andy,

Why not using RG174 BNC clamp connectors?

73 Ludwig

I acquired about 40 RG58 BNC crimp and 15 RG174 BNC crimp connectors for free. Buying a crimp tool made sense so I could use these connectors. The RG174 ones are Chinese but OK for QRP and HF. The RG58 were Greenpar or Radiall ISTR and should be OK for up to SHF even if the RG58 is lossy.

I have a whole load of RG58 BNC rubber boots if you need some.

Well I might take you up on that John.

Well done on making your first antenna. They get much easier to make with practice and also you begin to learn the difference between theory and practice as you make more. So you know how much to adjust lengths and whether a match that is only 1.4:1 needs improving or not.

I’m not sure on your background so you may know this already but where your wires join connectors is where you will get fatigue/stress breakages and that’s the place to make sure you have strain relief. The other is that using figure-of-eight winding (such as kite winders) results in less strain breakages and stops the wires getting twists in them and becominge a huge big knotted mess on the summit.

1 Like

No worries Andy.
They look like
image

Cheers, its the first i have made an antenna to TX/RX now I need test in anger.

Background is IT. I will have a look at the figure of 8 winding.

Cheers.

I like BNCs, but one thing to note - the centre pin is a fairly loose fit in the PTFE of the plug.

When you plug in, the centre pin is held relatively firmly in the socket.

So, if you then rotate the outer part of the connector - maybe to adjust where the cable lies relative to the controls on your FT817 for example - the outer of the plug rotates, but the pin doesn’t. This puts a lot of strain on the inner conductor of the coax where it meets the centre pin, and this is a common point of failure after repeated twisting. I’ve got into the habit of putting the cables where I want them before connecting up…

(Based on experience of fitting many hundreds of the things to thin ethernet, and fault finding when others have moved stuff around etc :o)

1 Like

This is just one of many kite winders available. It just happens to be from SOTAbeams and other vendors sell them.

1 Like