Tuning and balun on home made T2LT

I can't seem to get the SWR to an acceptable level, 2.5:1. I am not happy to run a linear with the SWR that high.

The dimensions are:
Element length = 2.950m.
Balan to element length = 1.640m
Coax balun on 11cm diameter stiff cardboard with 5 turns of Super 8 xx coax.

Would increasing the balun to 18cm help?
Would just raising or lowering the choke reduce the SWR?

I am waiting some HF antenna books so I could learn more but in the meantime thought I'd ask here.

TIA, Simon.

Hi Simon ....

I'm no expert, and stand to be corrected, but most T2LT "designs" I've come across use RG58 coax which may have a different velocity factor to Super 8, which affects the dimensions of the element etc; so that may be one thing to look at.

Another thing is the height above ground. I've seen that affecting the SWR of some people's T2LT antennas too. I have a vague recollection of it needing to be at least a 1/4 wave-length high.

And, finally, check your connections. I couldn't get the SWR down on my dipole until I sorted out my rather crappy first attempt at soldering a P-259 plug on to the feeder.

73s

Nick

https://www.youtube.com/watch?v=senQ5_fc5WU   VID by Gary CT(X)104

The dimensions look a bit odd. 5 turns on 11cm for the choke/balun looks as per the usual design, but the quarter wave sections - i.e. the coax above the choke, and the length of wire or coax inner above it would both usually be about 2.6m/102". The principle of operation is that the T2LT is a dipole, so the two halves of it should be about the same length - though it is usual to trim the top section to get a decent match.

The velocity factor of the cable shouldn't matter as while the "bottom element" is made up of the coax, the radiation is taking place from the outside of the outer sheath, which will not be affected by the velocity factor. The choke isn't actually performing any balanced/unbalanced transformation, it is there simply to trap the RF at a quarter-wave length down the coax to create what behaves like half a half-wave dipole.

26TC62 wrote:

The velocity factor of the cable shouldn't matter as while the "bottom element" is made up of the coax, the radiation is taking place from the outside of the outer sheath, which will not be affected by the velocity factor. The choke isn't actually performing any balanced/unbalanced transformation, it is there simply to trap the RF at a quarter-wave length down the coax to create what behaves like half a half-wave dipole.

The velocity factor is affected by no small part by the insulation of the cable so even though the radiation is happening from the outer sheath that is covered by insulation.

EVERYTHING that is in contact with the outer surface of a conductor, because RF flows on the surface of a conductor, affects the velocity factor. It is one of the reasons that wire antennas de-tune when it rains, even the water on the wire/insulation alters the VF of the wire.

Northern Crusader wrote:

The velocity factor is affected by no small part by the insulation of the cable so even though the radiation is happening from the outer sheath that is covered by insulation.

EVERYTHING that is in contact with the outer surface of a conductor, because RF flows on the surface of a conductor, affects the velocity factor. It is one of the reasons that wire antennas de-tune when it rains, even the water on the wire/insulation alters the VF of the wire.

That's true, but the VF of the outer sheath of any kind of coax is not going to vary as much between different types of coax as the spread of VFs of different kinds of coax operating as a coax cable - there is about a 30% difference in wavelength in coax between one with 0.66 VF and 0.95. For the lower half of a T2LT, the VF of the coax *as coax* is irrelevant. There is not going to be enough variance in VF of the outer sheath between different kinds of coax to make much of a difference. That has been my own experience having made them out of RG58, Mini 8, RG213 and even RG59.

Pappy Nick. Thank you, I have rechecked the soldering and it seems fine, I did use the vf for super 8, and Im sending it up a 10m fiberglass only pole so the height should be ok'ish, good call though.

Bean. Thank you also, I have watched the video and used the internet calculators and have got to the position I am now in. Your contribution however is useful as I have now bookmarked the page!

26CT62.

but the quarter wave sections - i.e. the coax above the choke, and the length of wire or coax inner above it would both usually be about 2.6m/102". The principle of operation is that the T2LT is a dipole, so the two halves of it should be about the same length - though it is usual to trim the top section to get a decent match. wrote:

Yes, that is what I thought and found it a bit weird that the dipole calculator gave differing lengths.
I also understand about the VF factor which has a part to play, thank you to Northern Crusader for expanding my knowledge a little further on this particular aspect.
Northern Crusader, I would be interested to see what effect covering the foamy centre element with the outer shell of "network cable waterproof sheathing" has on the swr when it gets wet. I understand the principle now of water getting into the coax. I would really like to use the T2LT on my 10m pole all through the winter to participate in the nets as an alternative to my trusty Tank Whip.

So, I am going out to the car right now to retreive the T2LT as the Mrs has said I am allowed to lay it out on the front room floor in front of the telly to make some adjustments.

Thank you all for helping me out.

Next projects are an Inverted dipole and then a Mag Loop once I find a suitably priced air capacitor and a friendly plumber who will bend my copper pipe.
I really have got a bad antenna bug, that's the beauty of this hobby tho'.

Thanks again all.
Kind regards.
Simon.
26CT2379
M7SMN

Dogfish wrote:

once I find ... a friendly plumber who will bend my copper pipe.

Ooh Er Missus !!!

UPDATE:

My T2LT is finally tuned in and ready to go.
I have decided to use it as a "stealth" antenna and put it right up in the top of a tree at my home QTH which is 3 times higher than my Sirio Gainmaster 5/8 antenna.

Thank you all for your helpful advice.

Kind regards.

Simon.

Dogfish wrote:

Northern Crusader, I would be interested to see what effect covering the foamy centre element with the outer shell of "network cable waterproof sheathing" has on the swr when it gets wet. I understand the principle now of water getting into the coax.

Yeah coax will wick up water like a sponge. The sheathing will alter the velocity factor of the "wire" and affect the tuning. I'd say to play with antenna modelling software like EZNEC which allows you to put in the dielectric constant of the insulation you're using if you're able to find the specs for it by creating a simple dipole with bare wire for say 28MHz then altering the insulation for the wires and seeing where the new resonant frequency is.

Antenna modelling software is a good way of learning how antennas work and how things like varying their height and length and things like nearby metal structures can alter the signal.

How did you finally tune it buddy