Broadband Dipole View

[David Crespo]

Hello, I need to change my J pole antenna, which although it works very well, I have had to change the frequency and I still don't know which will be the definitive one, since where I live it is difficult to find a free frequency. That's why I want to put a broadband dipole and thus forget about having to disassemble the antenna every time. I will use a 300W amplifier in the future.
I have seen this one and I wanted to know your opinion, what do you think of this type of antennas, thanks and regards!
https://todofm.com/product/dipolo-de-ba ... st-akf-1n/

[Albert H]

You're going to be very disappointed with that "dipole". It's more of a dummy load! The radiation efficiency is traded for bandwidth, and you'll never get a particularly good match from this thing except at its one natural resonant frequency. It's also ridiculously expensive!

Building aerials is actually quite easy, and if you've built a J-pole, you can actually make it tuneable - you have to vary the lengths of the ¼-wave and ¾-wave sections, but if you get the feedpoint bang on for 98.4MHz (the geometrical mean frequency of the band!), you can tweak the lengths for pretty good minimum SWR anywhere in the band.

My variable frequency J-pole used aluminium elements cut for 107.5 MHz, and then extended by screwing lengths of threaded studding. I ended up cutting half a dozen lengths of brass studding, and could cover the whole band with an SWR less than 1.1 : 1 by screwing in the various lengths. I wasn't too concerned about weather-proofing the aerial, since it was just used for testing purposes, but if I wanted to weather-proof it, I would have bought aluminium studding.....

I've also made temporary J-poles at various frequencies out of Meccano! I've got one in my workshop at the moment calibrated for the middle of DAB channel 5C (173.4 MHz), which I've used for a local experimental DAB rig. After tweaking it a bit in my garden, I got an SWR of almost 1 : 1, with practically no measurable reflection whatsoever.

[sinus trouble]

I doubt you will ever get a truly 'Broadband' antenna from experience!

But i guess as Albert mentioned, How can you maximise gain with the ease of tuning included?

I have heard that the folded dipole is 'Broadband' But dont quote me on that statement!

[Albert H]

Nope. The folded dipole has somewhat widened bandwidth, but has a feedpoint impedance of around 300Ω. You have to use a balun to convert the unbalanced coax to the balanced feedpoints of the folded dipole, and to convert the impedance up from 50Ω to 300Ω. Almost any kind of transformer (balun) is going to be lossy..... You might win some bandwidth, but you'll be wasting power as heat!

We used to supply a quad co-phased loop dipole antenna. It had (about) 4 MHz bandwidth and the most complicated phasing harness I've ever seen. The losses in the harness were so bad that a quad of straight dipoles radiated just over 2 dB more! I gave them up (despite them being conveniently shortened) and built a few quad-stacked dipoles, which were taller aerials, but radiated so much better!

If you "fatten" the dipole elements, the bandwidth increases, but - again - at the cost of radiation efficiency.

If you want to make an adjustable aerial for testing purposes, get a couple of telescopic rod aerials, a small plastic box, a FT-114-67 toroidal core, and make a transformer- fed dipole that you can tweak the length of! You need to be able to make each element 66cm for 108 MHz out to 82 cm for 87.5 MHz. https://www.amazon.co.uk/KESOTO-Replace ... B07H725YMX will do the job, and you get two of them for £8.

[David Crespo]

Thanks for the opinions. If you think this dipole is expensive, it is the cheapest I have found.
The problem is not building an antenna that I have already done, what I don't want to do is disassemble the antenna every time I need to try a new frequency, it is also on a community terrace and I don't want to bother it too much. I also don't mind if I lose some power, I don't need to go very far.
There is also the fact that I want to put about 300W in a few months, and I don't know if a copper tube j-pole would support that power.
I'll think about what I can do, thanks!

[Albert H]

I've run over 1 kW into copper pipe home made aerials. My favourite is the "copper cactus", which is two co-phased J-poles. It gives a lot of antenna gain, and is not too hard to construct and align.

[rigmo]

Albert do you share any photo ?

[radium98]

@rigmo here it is

[rigmo]

@rigmo here it is

co-phased ?

[Prism]

Screenshot_20220130-124946_DuckDuckGo.jpg

what about this ?

[sinus trouble]

I do like Alberts approach with the adjustable screw in studs!

To stress once again! I doubt you will get perfect performance across the band!

Another point to add? Its not really good for your listeners to be all over the band from one day to the next?

I would suggest you carefully choose a frequency and stick to it!! No matter the cost!! That is the Pirate way!!

[ronald001]

I have seen this one and I wanted to know your opinion, what do you think of this type of antennas, thanks and regards!
https://todofm.com/product/dipolo-de-ba ... st-akf-1n/

This antenna is well known among the dutch broadcasters, and there is nothing wrong with it.
According to the specs the SWR should be <1.35 across the whole band, and gain 2dBd compared to a half wave dipole.
I doubt the gain, a tuned antenna should give better results - but overall it's not a bad antenna when you need to change your frequency now and then..

[Albert H]

They're ridiculously expensive! And the "gain" is pure fiction....

Ze zijn belachelijk duur! En de verstering is pure fictie....

[ronald001]

Ze zijn belachelijk duur! En de verstering is pure fictie....

Haha true, gain is a bit too much to be true - and for a piece of welded aluminimium it's indeed a bit pricy.
On the other hand, if you cannot weld and cannot make it yourself - it's the cheapest broadband antenna you can buy...

[yellowbeard]

That is expensive, there is no denying it. If you're not going to make your own I would consider this better value, and tunable when you are ready. Bandwidth is over 6MHz with an SWR of under 2:1 once set up. You can get a similar performance out of a slim jim antenna made from 12.5mm copper pipe for around half the €37.50, but this does look better and less fiddly.
https://www.sirioantenne.it/en/products/vhf/gpa-series
https://dutchrfshop.nl/en/antenna/111-s ... meter.html

[David Crespo]

Hello, changing the frequency is not for pleasure, but because they put more powerful stations with which you can not do anything and you have to change, also in my area there are no free frequencies.
Sririo had recommended this one to me but I see that it has a very high SWR: https://www.sirioantenne.it/en/products ... -lb-series

[mikroman]

I had experience with the GP-LB series. Significantly shorter range than with ordinary dipole. Poor VSWR on this model can be corrected by shortening the radials. Another thing that does not match the characteristics of the manufacturer is wind resistance. Two radials flew at 90km / h. I had the opportunity to mount their gamma match dipole. It's mechanically ok, except that part of the system is a plastic insulator instead of teflon, so it melts at 200w in the summer months. There is also corrosion of the nuts in the connector-gamma chain. It should be taken into account that there is a UHF connector, so you need to additionally protect the connector from moisture. In short, it is worth exactly how much it costs.

[rigmo]

Screenshot_20220130-124946_DuckDuckGo.jpg what about this ?

broadband extremely good and possible get perfect match if you need .