best antenna for me?

[The DB]

Is common mode even an issue on UHF though?

For the t2lt antenna design he is looking at? I would say yes.


The DB

 

[The DB]

Similarly not being an amateur and having no interest in the very expensive 934 Mhz we had back in the 80's which was barely affordable and lacking equipment for amateurs was never going to be cheap in the 80's.

The ones I have seen used have had tighter windings than your average top end of hf ones, but intuition tells me not tight enough and far far too many turns, I have obviously considered clip on ferrites as an alternative, I plan using a cushcraft ARX 450 for the job as with a great lump of basalt volacanic plug blocking my main target area and a quarry blocking me to Glasgow my home town, supplied handi antenna and upgraded na771 just don't cut it from indoors,

I have to walk to river front to get round both, I have made the trip from there but even then signal is touchy and just turning round or moving slightly can destroy signal, I generally listen for best incoming signal and obviously that will be reciprocated on the outgoing side, not sure if I can access roof of 3 storey building, but do have a satellite dish outside I don't use and could fit the Ringo Ranger there, but although I know its not ideal may be only option, I'm also aware RG213/U isn't the best option at that frequency either,

Hardline heliax with N plug would be a far better solution but very costly and even then I might still not achieve my objective, might not even be achievable from chimney or 10 foot mast on top of it, its more for curiosity than anything else,

I realise the bend ratio may not be achievable with any coax and line isolater / ferrite clip ons may be a far more productive solution as common mode current will almost definately be a certainty with that antenna, I really do need to get a vna or mini vna that can go that high or at very least an antenna analyser, I know mfj 269 will go there but it ain't gonna be most accurate tool for the job, although might be only semi affordable one, It could also be used for building an efficient T2LT as well, so it could come in useful to many people operating up there, even a swr meter for that frequency range is pretty expensive, I really just wanted confirmed my suspicion that the chokes I've seen used which resemble that on the gainmaster are not effective for the job, thanks for taking the time to answer me, I would imagine dimensions wouldn't be too far off a similar one for the amateur 70cm's band but have struggled to seek info even on that, I'm thinking same as you ferrite clip on/s is deffo way to go but was curious if it was possible with coax,

it is just a gut instinct from what I know about HF chokes that what I have seen just can't be effective as those types of chokes are very narrow banded and without sufficient choke impedance could either be useless at best and detrimental at worst.

Thanks for your effort, it is much appreciated, Jazz 73.

I've done some research on-line on these, and there really isn't much in the way of straight forward information. Typically they say anywhere from 4 to 6 turns without a form size.

Of the few that do give coil diameter sizes most say to use a 4 to 5 inch diameter plastic tube... 4 to 6 turns on a 4 to 5 inch diameter tube will center the choke in the upper HF range to lower VHF range at best, unless they are thinking a harmonic of the resonant frequency is going to do the job for them. If that is the case, it would be an awfully distant harmonic, and as such would have limited effect at best, especially an antenna whos design is partly dependent on common mode currents...

One site said to use 5 turns on a 1 inch diameter form. This was for a 2 meter/440MHz antenna, and offhand I would think that would be closer to the 2 meter frequencies, although I'm not sure. I don't see how it could be effective at both sets of frequencies as they are just to far apart...

One site made a current balun for their choke using ferrite and two wires. I think this design is the only design I've seen that would effectively cover both bands of the multiband antenna as that particular balun design is effective and efficient over a large frequency range.

With all the varying and questionable data out there, perhaps this would be a job for modeling. I know your feelings on modeling, but it should at least get us in the ballpark. That being said, I would be pushing into areas of modeling I have never attempted in the past...


The DB

 

[jazzsinger]

George, you can usually find lengths of second hand hard line heliax at radio rallies and its all perfectly usable. A friend of mine has accumulated quite a lot of quite decent lengths over the years, all with those big fat expensive N-type connectors on the end.

As for the chokes I think you're basically going to be bifilar winding on a suitable mix ferrite core whatever that mix would be. Is common mode even an issue on UHF though?

Hi Conor,

I'm familiar with radio rallies and what is usually available, they are generally pretty shit here, even the biggest one at Magnum Centre in Irvine is shit, I just wish I'd had money when cut off's of proper mil spec RG214/U was on sale on E bay, but I was a bit tight at time with moving house, should have bought the lot as it was cut offs off about 100m and reasonably priced, it came from a commercial install.

Common Mode is a huge issue with this antenna as DB says, it sets length of lower half of dipole as well as choking the shield from radiating, although the radiation/noise pick up may not be a huge issue at uhf, the tuning is critical. 73 Jazz

 

[jazzsinger]

I've done some research on-line on these, and there really isn't much in the way of straight forward information. Typically they say anywhere from 4 to 6 turns without a form size.

Of the few that do give coil diameter sizes most say to use a 4 to 5 inch diameter plastic tube... 4 to 6 turns on a 4 to 5 inch diameter tube will center the choke in the upper HF range to lower VHF range at best, unless they are thinking a harmonic of the resonant frequency is going to do the job for them. If that is the case, it would be an awfully distant harmonic, and as such would have limited effect at best, especially an antenna whos design is partly dependent on common mode currents...

One site said to use 5 turns on a 1 inch diameter form. This was for a 2 meter/440MHz antenna, and offhand I would think that would be closer to the 2 meter frequencies, although I'm not sure. I don't see how it could be effective at both sets of frequencies as they are just to far apart...

One site made a current balun for their choke using ferrite and two wires. I think this design is the only design I've seen that would effectively cover both bands of the multiband antenna as that particular balun design is effective and efficient over a large frequency range.

With all the varying and questionable data out there, perhaps this would be a job for modeling. I know your feelings on modeling, but it should at least get us in the ballpark. That being said, I would be pushing into areas of modeling I have never attempted in the past...


The DB

Hi The DB,

Yeah I've seen the 4 or 5 turns at 4-5" diameter, Ideal for CB and possibly 10m, but I'd imagine completely useless at UHF, I think some people read something and apply the law that as antennas can be scaled then chokes will do them all, which as we both know is far from the case.

Ham universe has a lot to answer to when it comes to misinformation about chokes as Steve G3TXQ has proven, he might just be the guy to talk too, but been a while since I was on HRD, where he often sits, I've had a few chats with him in the past and he is a very knowledgeable guy, of that there is no doubt.

I also saw the 5 or 6 turn at 1" diameter, soon as I saw it I thought far too many turns and impossible to achieve that bend ratios on all but the thinnest lossiest coax,even they might struggle to achieve 1" diameter and not ideal for UHF as losses would be incredible.


I was thinking along the lines of 1 turn at about an inch diameter myself, but TBH, I'm not sure even that would do it and certainly not feasible with RG213/U's bend radius. I think an air wound choke is just unfeasible, period.

I'm thinking like you, clip on ferrite/s may well be the answer and a damn sight tidier too, If only I had a VNA that went that high to test it, I have no doubt it is possible and essential, but how many would I need, one or two, a whole string, the whole coax covered? You see where I'm coming from I think.

On modelling software I have grave doubts, but in saying that I am open to everything, Only a fool would say that's shit without thorough investigation, I see holes in it for modelling real world antennas and real world ground but that's not to say it couldn't do this job. Obviously a VNA of the coverage required would be ideal to determine this, but I'm guessing well out of my price range.

Thanks for your input, you've basically confirmed what I suspected that it is indeed unfeasible and those using them don't know about RF Theory but merely took something that worked elsewhere and transposed it on a frequency it most definately won't work on. I have no doubt a T2LT on 10m fibreglass mast will perform much better than an 11m one on same mast (as its relative Lambda above ground is far greater) which has proven to be quite effective here, but skips good, will it still be effective when solar minima appears and all the A99 users start to wonder who switched off the skip, lol

Think I need to start reading up on clip on ferrites, I know they work at HF, basically what a line isolater is inside a tube, but how effective are they on UHF and what mix would be best is the things I need to explore, I am very surprised that many amateurs have not done this with 70 cm being so close, I guess most buy shop bought antennas at that frequency or convert TV Yagi's and hope for the best.

Guess I'll just need to hope either yourself or someone else who has the test gear to measure for me could do it, think the Ringo Ranger will need it as well, but that's another story, maybe just trial and error will get me close if I can borrow an MFJ 269 or better, I know how to go about it, its just the lack of measuring gear that is holding me back, Even when I did have good gear it was never aimed at this high.

Thanks for your time and thoughts, if anyone else has any thoughts on the subject outwith those already mentioned I'd be interested in hearing them Thanks a lot, Jazz 73

 

[northern35s]

I asked Steve how he set about testing chokes, and apart from remembering he use an AIM4170 analyser I can't remember the details, he did say that a two port analyser would be easier, I'll see if I can find the details and will post it here when I do.

 

[northern35s]

Found this series of posts by Steve, G3TXW, on HRD (HamRadioDeals)

Firstly, you can easily achieve 5000 Ohms choking impedance with an air cored coil of coax.

Secondly, the inductance formula is pretty irrelevant. The stray capacitance across the windings turns the choke into a parallel tuned circuit rather than a simple inductor. What matters is its self-resonant-frequency - if you can get the SRF to match your operating frequency you will have a very high choking impedance.

The problem with air-cored chokes is that they are quite narrow-band and the choking impedance falls away rapidly either side of the SRF. A better choice for HF and low VHF is a ferrite-cored choke - it maintains the choking impedance over a wide band and is much more forgiving of getting the number of turns wrong.

The only calculator I know that takes into account the winding capacitance and attempts to predict the SRF is here:
RF Inductance Calculator - HAMwaves.com

Even then I prefer to just measure it

Just made some SRF measurements - all with RG58 close wound on 53mm diameter form:

12 turns 48.3 MHz
11 turns 50.5 MHz
10 turns 52.5 MHz
9 turns 56 MHz
8 turns 59 MHz
7 turns 64 MHz
6 turns 70.4 MHz

73,
Steve G3TXQ

A GDO is the most reliable way. We're talking about winding capacitance of less than 1pF in some cases, and anytime you connect measuring equipment directly to the choke the equipment capacitance makes the measurement invalid.

An air cored choke is just about OK for dual band VHF operation provided you wind the choke to get the SRF between the two bands and provided the application is "benign" - by that I mean you're feeding a well-balanced antenna with a reasonable 50 Ohm match.

The problem is that if you took my figures, but altered the diameter or the coax size you'd have a quite different SRF!

Steve G3TXQ

Here Steve answers a similar question:

Steve, on your site you have that coloured chart box it gives various impedance readings for air cored/ferrite chokes, how did you get those readings, is this something I could do with my balun ?


You need to be able to measure complex impedances with very high values. Analysers like an MFJ259B wont get close; an AIM 4170 will do OK for the low-Q ferrite chokes; but for accurate measurements on Type 61 ferrite chokes, or air-cored chokes, you really need a two-port VNA. In the past I've used a spectrum-analyser/tracking-generator combination, and also used a signal-generator/RF-millivoltmeter combination, but it's a bit tedious doing the maths with either of those.

The basic problem is the stray capacitance added by the measurement instrument and/or test jig. The choke itself may be self-resonant because of typically 2pF of "internal" stray capacitance; if you now add, say, 0.5pF of external capacitance from the measurement instrument or test jig, a choke which is actually self-resonant at 21MHz will appear to be resonant at 18.8MHz.

For the most accurate results I use a 2-port VNA with a test jig that I measure has a capacitance of about 180fF

73,
Steve G3TXQ

Just found the one where I asked about his test fixture:

If your choke is wound with coax, simply connect the braid from either end of the choke across the analyser; if it's a bifilar winding, connect the two ends of the same wire across the analyser.

If you have something like an AIM4170 analyser you'll be able to "calibrate out" the connection terminals and get reasonably accurate results on the "lower-Q" broadband chokes.

However, I find the AIM4170 still adds a few pF to the measurement, and that can cause a significant error on the Type 61 ferrite and air-cored chokes. For those I now use an S21 measurement using a 2-port VNA which adds about 0.2pF across the choke.

Hope that makes sense!

Steve G3TXQ

This is the arrangement I use for 2-port VNA measurements:

Firstly calibrate the VNA by lifting the two coax tails (from Points A&B) and connecting them together.

Then connect the choke between A & B and make a measurement of S21 Amplitude and Phase.

Then:

Zchoke = 50*(10^(-S21amp)/20) - 50

Rchoke = Zchoke*COS(-S21phase)
Xchoke = Zchoke*SIN(-S21phase)

The 2-port method gives more accurate results and can cope with much higher choke impedances.

73,
Steve G3TXQ

 

[Beetle]

I just bought a Magnum S-6-175+. it came with a magnetic Wilson 1000 watter. I only tested it on the way back home in my Dodge Durango. It does work well with the Wil 1K but wanted to know some professionals opinions.
Also fill me in if dual antennas is better?

I will be slowly tweaking my system to get the most out of it as I can. Not looking to add another amp or anything just a good tweak on the radio.

"Best antenna for me" is something only YOU can determine.