Spacing of 2m / 70cm yagi's

[VK1PWE]

Hello all. After much consideration for the best (for my situation that is) portable configuration, I've decided to go with yagi's on 2 and 70.

Can anyone advise what the spacing should be between the 2 and 70 yagi's? I recall seeing 6ft is required, but many web sites with photo's of portable ops show a spacing of much less - as little as a foot in some cases.

With a 10ft mast, 6ft is not ideal - I'm hoping about 4ft would be okay. 2m yagi at 6ft and 70 on top.

Grateful for any advice...

[VK3HZ]

Hi Scott,

Not sure what other type(s) of antenna you were considering, but for portable operation, you really can't go past yagis for ease of transport, low wind cross sectional area, ease of mounting, etc. Perhaps a dish for 13cm and above.

For spacing two yagis for different bands, the minimum is half the stacking distance of the highest frequency yagi. More (up to full stacking distance) is marginally better as, at the minimum spacing, the 70cm yagi pattern will be skewed upwards slightly by the presence of the 2m yagi looking like a ground plane - not really what you want on a mountaintop.

If you don't know what the 70cm yagi stacking distance is, then, for example, a 3m boom 16 element DL6WU design needs roughly 1.2m stacking distance, so can be spaced a minimum of 600mm (2 feet) above the 2m yagi. Longer/higher gain needs more distance.

Dave
VK3HZ

[VK1PWE]

Hi Dave, many thanks for the info. It changes daily, but today at least, I intend to use 10el on 70 and 5 on 2.

From what you've said, I think the 4ft spacing I will have will be adequate and sounds like it would be above the minimum required, but happy to be corrected there.

As for the other antenna I was considering, I was hoping to get away with a tri band vertical on 6/2/70 (Comet GP15), but after comparing my F23A with my 5el on 2 the other morning, the yagi wins hands down. It was always going to be the case of course - it's just I'd never compared them together at the same time. The F23 worked well, just not as good as the yagi.

Thanks again...

[VK1BL]

Dave Olean at DirectiveSystems in the USA has a great application note entitled "How to plan the installation of multiple VHF antennas on the one mast", go to http://www.directivesystems.com and navigate to "App Notes".

Dave also has some very good info on the effects of coax at higher frequencies and SWR issues etc. A worthwhile site for VHF and above antenna info...

73,

Ted VK1BL

[VK1BL]

Scott, in a portable situation I tend to stack the other way round. That is, put the 2m yagi at the top of a 4m aluminium pole then the 70cm yagi below that and the 23cm loop (end mounted) below that.

If you use the stacking distances suggested in Dave Oleans application note then there's likely to be minimal effects to the higher freq antennas. Stacking big at the top to smallest means less chance of physical restrictions/contact when you want to turn the whole arrangement...

But then in my case it's an evolving thing... )

73,

Ted.

Hello all. After much consideration for the best (for my situation that is) portable configuration, I've decided to go with yagi's on 2 and 70.

Can anyone advise what the spacing should be between the 2 and 70 yagi's? I recall seeing 6ft is required, but many web sites with photo's of portable ops show a spacing of much less - as little as a foot in some cases.

With a 10ft mast, 6ft is not ideal - I'm hoping about 4ft would be okay. 2m yagi at 6ft and 70 on top.

Grateful for any advice...

[VK2OMD]

Dave Olean at DirectiveSystems in the USA has a great application note entitled "How to plan the installation of multiple VHF antennas on the one mast", go to http://www.directivesystems.com and navigate to "App Notes".

Dave also has some very good info on the effects of coax at higher frequencies and SWR issues etc. A worthwhile site for VHF and above antenna info...

73,

Ted VK1BL

Ted,

The article you reference is a classic Aperture Area explanation of antenna separation. The suggestion is that an antenna viewed from an irradiating source presents a physical area within which the received power is collected or captured.

A practical observation is that high directivity antennas are less affected (pattern, gain, driving point impedance) by nearly off-boresite objects than low directivity antennas, yet the Aperture Area of the high directivity antenna is larger than the low directivity antenna.

The translation of the concept of a notional Capture Area (or aperture) representing an antenna's coupling to space (its ability to couple an amount of power from an incident wave, and to transform it to power in its load) to a physical space that defines the boundary of the antennas influence doesn't work well in general.

Owen

[VK2OMD]

Scott, in a portable situation I tend to stack the other way round. That is, put the 2m yagi at the top of a 4m aluminium pole then the 70cm yagi below that and the 23cm loop (end mounted) below that.

If you use the stacking distances suggested in Dave Oleans application note then there's likely to be minimal effects to the higher freq antennas. Stacking big at the top to smallest means less chance of physical restrictions/contact when you want to turn the whole arrangement...

But then in my case it's an evolving thing... )

Ted, have you thought about locating the 23cm antenna between the 2m and 70cm antennas, and closer to the 2m antenna?

Owen

[VK2OMD]

Hello all. After much consideration for the best (for my situation that is) portable configuration, I've decided to go with yagi's on 2 and 70.

Can anyone advise what the spacing should be between the 2 and 70 yagi's? I recall seeing 6ft is required, but many web sites with photo's of portable ops show a spacing of much less - as little as a foot in some cases.

With a 10ft mast, 6ft is not ideal - I'm hoping about 4ft would be okay. 2m yagi at 6ft and 70 on top.

Grateful for any advice...

Hi Scott,

This is a contentious subject. There is much written on stacking distances for arrays of yagi antennas for the same band. Some people extend the "advice" for that scenario to the "different band" situation. The reasons proposed (if indeed any are) are often questionable.

My recollection is that the ARRL Antenna Book reccomends half the boom length of the smaller array for "different band" antennas, and then goes on to say similar spacings are appropriate to "same band" antennas.

There are certainly Rules Of Thumb (ROT) bandied about for same band antennas, like 50% and 5/8 of the boom length, and people wax on about the relative merits.

At http://www.vk1od.net/dl6wu/StackingROT.htm I have plotted the recommended stacking distances of two DL6WU arrays on 432, vs boom length. You will see that any x% of boom length estimator is doomed to fail in the wider context. That plot might make you sceptical of simplistic ROT.

My own thoughts are that for the different band scenario, whether the antennas are harmonically related is important (and they will tend to be on the bands above 50MHz), the nature of that relationship (eg 3:1), the wavelength and beamwidth of the antenna in the stacking plane, the height above ground or other objects will all influence the answer.

The graph above was derived from the work for the article http://www.vk1od.net/dl6wu/EstimatingBeamwidth.htm .

NEC models of some hypothetical scenarios would be interesting.

Sounds like subject matter for some interesting experiments.

Owen

[VK1BL]

Ted, have you thought about locating the 23cm antenna between the 2m and 70cm antennas, and closer to the 2m antenna?

Owen

G'day Owen, I guess if I were to extend my thinking in accordance with the model presented in Dave Oleans article then it makes sense to do just that in order to further reduce the space consumed by the 3 yagi stack...

Is that what you're suggesting?

Ted

[VK2OMD]

Ted, have you thought about locating the 23cm antenna between the 2m and 70cm antennas, and closer to the 2m antenna?

Owen

G'day Owen, I guess if I were to extend my thinking in accordance with the model presented in Dave Oleans article then it makes sense to do just that in order to further reduce the space consumed by the 3 yagi stack...

Is that what you're suggesting?

Ted

Hi Ted,

Firstly, the "rules" for separation of stacked "same band" antennas that are effectively paralleled at the feedpoint are typically to achieve an increase in aperture based on interferometry. When you explore what makes interferometry work, it shows why optimal stacking distance is related to the pattern of the single antenna and the expected outcome of stacking. It happens that the first null of a high gain yagi is approximately twice the half power angle. When you consider that the stacking objective is to achieve about 3dB increase in gain by doubling the aperture in the stacking plane, then the new half power beamwidth in that plane will be half the original, and the new null will be half the original and equal to the original half power angle. Many of the "sane" stacking distance formulae find the distance (or a small variation) that delivers this result. Interferometry is not relevant to separation of "different band" antennas, and recommendations for separation of "different band" antennas based on "same band" antennas don't make sense.

Determination of separation of "different band" antennas is typically to achieve isolation (eg for protection of a receiver on the non-energised antenna) and reduce interaction (change in feedpoint Z, pattern distortion) to an acceptable level. Interaction will depend on mutual coupling factors like separation, harmonic relationships, polarisation etc. The increase in isolation with distance should be greatest in the induction near feild region of antenna elements (which is generally taken to be within a radian distance of the element). At greater distances, the isolation increases at a decreasing rate until it reaches the inverse square law where the distance from the element is greater than a radian or so and the angle subtended by the source is small. In simple terms, isolation should increase very rapidly until about 1/6wl separation whereupon it will slow. It may be possible that sufficient isolation wrt pattern distortion is achieved at distance that does not provide sufficient isolation of a receiver on the non-energised antenna and either filtering or some other method of protecting the receiver is necessary.

Owen

[vk1da]

Hi Scott,

Practical limitations when you are portable mean you have to accept some compromises.

When you connect the antenna and signals are coming in, you just work as many as you can.

The theory helps you avoid tiny spacings of several inches, but optimal spacing probably doesn't matter much given how many other variables there are. It's more important to get it up, get on the air and work what you can. Learn as much as you can from each setup.

Use good cables - even a 6m cable has losses which are real on 70cm.

Hope you will continue to operate in the field. We have a good trend going in vk1 at present, let's keep it going.

Andrew
VK1DA