Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

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John Faulkner, Skegness
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Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by John Faulkner, Skegness » 27 Apr 2015 01:32

Since my 9.2 antenna now has a dedicated band 2 pre-amp at the mast head, I can no longer use this for OIRT reception, so I will build a simple half wave horizontal dipole for this range.

My question is: Would a half wave dipole, cut for 70 MHz, have any detrimental interaction with the 9.2? I'm sure this was mentioned somewhere and I seem to remember this was OK, so just to be sure.

The 70 MHz dipole would be roughly 1.5 metres below the 9.2.

I might even make it a two element if I can make it wideband to cover the whole range between 65 - 75 MHz if I can find the dimensions.

Thanks.

Andrew Webster,Wigan
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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by Andrew Webster,Wigan » 27 Apr 2015 12:33

I wouldn't think the dipole would have any interaction on the 9.2 I have an FM dipoles and and at one point 2 of them less than 1 M under the FM 5 and had no undesirable effects.

I would try it and if there are any undesirable effects take it down again but I will bet there won't be any problems.

Andy
Receiving equipment: 2 Sony ST-S311, 1 Sony ST-SE520, 1 Sony ST-SE700 2 crossed FM 5s rotatable, 1 homemade FM 3 horizontal beaming S/E, 1 vertical FM 3 fixed beaming at Ireland, 2 element band 1.

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John Faulkner, Skegness
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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by John Faulkner, Skegness » 27 Apr 2015 12:42

Thanks Andy.

As the OIRT antenna is at a different frequency, and there are no frequency multiples involved, I guess any interaction would be minimal. Sporadic E will still push though. But I am sure somebody did some computer modelling on this very situation. I am hoping somebody still has this data.

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by satnipper » 27 Apr 2015 13:07

I don't think there would be any issues - you're not exactly bothered about any f/b degradation for OIRT, and a marginal loss in gain would be undetectable. The transmitting paradigm might have been more troublesome with high swr but that's not relevant. Link here for some interaction modelling but not your exact situation - http://www.g3tct.co.uk/antennas.html

I did buy Steve Nicholls "Introduction to antenna modelling" but I'm keeping it until after I retire.

Stuart

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by John Faulkner, Skegness » 27 Apr 2015 13:24

Thanks Stuart.

The difference between those to antennas looks negligible to say the least and not worth worrying about.

Any OIRT dipole I use will be fixed to the mast, so not rotatable. I'll probably keep it to a single element for now.

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by daveB » 27 Apr 2015 17:19

Hi John,

According what I've modelled on 4nec2 you can get a reasonable 3 ele 64-76 MHz aerial on a 4 foot boom. The driven element has a short parasitic element 2.5 inches in front of it to give a wider bandwidth. The model says it's got 12-15dB f/b and 6dBd gain (sounds a bit high to me). A two element aerial - with reflector driven ele/parasitic ele gives around 7-10dB f/b ratio - so to me it would not make sense to go for a 2-ele model

So for a single element dipole - using a standard black dipole centre for half-inch tubing a half element length 1 metre is just about OK but 1050mm is better giving a 64-76 MHz bandwidth at less than 2.5:1 SWR. This allows for the fairly wide gap between the element ends

But if you then stick a parasitic element with a total length of 1220 mm in front of it with a spacing of 63.5 mm (2.5 inches) you get a dipole with a bandwidth of 64 - 107 MHz within 2.5:1 SWR limits. Why might you want to do that - well if you want to set up background scanning for early warning of openings then you can sweep both bands with the one aerial?

You could go for a 60-108 MHz wideband aerial - f/b ratio is 10dB-7.5dB in the OIRT band and 8-11dB in the CCIR Band. Obviously rubbish compared to a Kroner, but again someone might want a wideband aerial for background scanning to include the main FM band and switching ver to their main aerial.

Here are the dimensions - perhaps Brian K6STI might like to check them out and improve before anyone else tries to build one. I modelled in free space with 0.5 inch dia elements and 1 inch diameter boom - with an air space of 0.25 inches between boom and elements. I've had to post them as a jpg as I can't work out how to include a formatted table!

For 2 ele version just leave off the director and accept 3-4dB less f/b ratio
OIRT Aerial dimenions.jpg


Rgds

David
SDRPlay, AirSpyMini, RTL dongles, SB920s, Yamaha TX930. 5 ele compact yagi on rotator at 18ft agl. FM5 facing east, FM5 facing south, FM3 facing NE, OIRT dipole, 3-ele Moxon vertical facing east.

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by John Faulkner, Skegness » 27 Apr 2015 22:16

Thank you Dave. Very helpful indeed.

I will probably just try the single dipole on the mast for now as it will be simple to do and it will be hidden out of view. At a metre or so above the roof that should work well enough. I don't plan to spend a lot of time on the OIRT band as I will be jumping very quickly up into band two the moment the MUF gets there.

I think a three element would be pushing my luck too much regarding planning. A three element tucked away somewhere sounds good in theory, but I don't think there is anywhere I would be able to hide something this large so easily. I still worry (far too much, I know) about potential problems with planning and even neighbours. I've been there and so I automatically expect the worst, but I may well build something temporary for the middle of the season.

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by k6sti » 12 Nov 2015 13:35

compg.png
compf.png
John, it's been quite some time since you inquired, but I just ran across this thread and I wanted to respond. I modeled a 2m-long dipole made of 10mm tubing 1.5m below the 9.2 boom. I shorted the dipole feed terminals.

Here's my recommendation from http://ham-radio.com/k6sti/notes.htm:

Stacking Dissimilar Antennas

Unless its elements are orthogonal to and bisect FM antenna elements, mount any antenna designed for a frequency between 50 and 150 MHz on a separate mast. Outside this range, stack the booms at least ten feet apart. Whenever possible, model a stack instead of relying on these general guidelines.

Brian

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by k6sti » 12 Nov 2015 13:55

compg.png
compf.png
Here I added a curve for a split dipole. This is approximately what you'd get when the far end of the dipole feedline is open and its length is a multiple of a half wavelength in the middle of the FM band. It's better than a shorted dipole, which is equivalent to using an open feedline an odd multiple of a quarter wavelength long, but it still affects the FM response. To eliminate all interaction, mount the dipole parallel to the 9.2 boom.

Brian

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by John Faulkner, Skegness » 12 Nov 2015 18:39

That's a good point regarding mounting in parallel with the boom - something I had not considered.

Thanks for this Brian.

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by John Faulkner, Skegness » 05 Apr 2016 22:46

OK, it's time to put this plan into action.

I have considered various options and I have decided to keep things simple. I will either mount the new two element antenna on its own temporary telescopic mast (likely) or just clip the antenna to the 9.2 mast and accept any interactions which might result. I don't mind any little quirk which will be thrown up due to the antennas being so close. But if interactions are severe, the antenna would be moved to a temporary mast.

So, two questions.

1. I need the dimensions and spacing for a two element antenna which cover 55 to 85 MHz. (In fact, I might try a three element as I'm sure I have enough aluminium.)

2. Would it be better to mount the new antenna on a wooden boom?

Question 2 will probably show my ignorance when it comes to the technicalities of antennas. On the other hand, perhaps this might be a good idea?

Thanks.

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by Andrew Webster,Wigan » 06 Apr 2016 12:25

Here are the dimensions of my band 1 aerial if this is of any use.

Dipole 275 cm
parasitic element 223 cm
Spacing 7.5 cm

I bought this from Southwest Aerials 40-70 MHz in 1980 and it works very well for band 1/OIRT and it is a compact aerial.
I would say you could shorten the elements to achieve 50-80 MHz I do have to say it's a very simple aerial no baluns but it works.

Andy
Receiving equipment: 2 Sony ST-S311, 1 Sony ST-SE520, 1 Sony ST-SE700 2 crossed FM 5s rotatable, 1 homemade FM 3 horizontal beaming S/E, 1 vertical FM 3 fixed beaming at Ireland, 2 element band 1.

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by John Faulkner, Skegness » 06 Apr 2016 13:24

Thanks Andy.

I'll need to shorten the lengths slightly to change the coverage.

That extra 3 or 4dB from using a 3 element might be useful though. Still undecided.

I'm going to adapt my old 6m beam to make the antenna. It has telescopic sections, so should lend itself nicely to being converted to cover a different frequency range.

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Re: Any interaction likely between Körner 9.2 and a dipole cut for 70 MHz?

Post by daveB » 07 Apr 2016 12:22

I was playing with the idea of a better OIRT aerial - not that I can actually install it - but it was instructive to model it using the 4nec2 program. All figures are taken off of the computer model - which was modelled in free space and with all elements insulated from the boom.

This is a 3/4 element aerial with a boom length of only 4 feet - as that's the length of boom I had.
Performance:
SWR between 65 and 75 MHz is 1.6 or less.
F/B ratio 13 dB at 65 to 18 dB at 75 MHz.
-3dB beamwidth is greater than 60 degrees
Gain is 6dBd

Dimensions as follows:
Reflector = 94 inches
Driven ele = 88 inches - mounted 24 inches from reflector
Parasitic = 72 inches - mounted 27.5 inches from reflector
Director = 68 inches - mounted 46 inches from reflector
Material - half inch aluminium

The element clamps I have use metal fixing bolts -but if I built it I would substitute nylon bolts instead.

The aerial still works as a 2/3 ele if you just leave off the director making a very short aerial but you lose out on f/b rati which drops to 10dB at 65 and only 8 dB at 75 MHz.

David
SDRPlay, AirSpyMini, RTL dongles, SB920s, Yamaha TX930. 5 ele compact yagi on rotator at 18ft agl. FM5 facing east, FM5 facing south, FM3 facing NE, OIRT dipole, 3-ele Moxon vertical facing east.

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