Effects of boom sag in antenna gain

The following discussion about the effects of boom sag in antenna gain took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.

On 25-Sep-2001 EA1ABZ wrote:
My 12 element BQH yagis booms are supported by ropes to avoid boom sag, but I have noticed that when it rains ropes become larger and boom sag occurs, so, that is my question: Has anybody measured or modelled the possible effect in antenna gain due to boom sag?

Any suggestions are welcome.

On 25-Sep-2001 SM5BSZ wrote:
Boom sag is not extremely critical. The phase and amplitude of the current on one element is mainly determined by the radiation from it's nearest neighbours and therefore some boom sag will not affect the current distribution much.

The way the radiation from each individual element sums up in the forward direction is not affected at all because the distance to a point far away in the forward direction is unchanged with some boom sag.

Perpendicular to the boom the distances to different elements will change quite a bit and therefore cancellation will be less good so some radiation will be lost in increased lobes far away from the forward direction causing loss of gain and degraded G/T.

A phase error 90 degrees away from the boom of +/- 45 degrees (wl/8) is approximately what one can tolerate. An old experimental observation is: "The antenna is ok as long as a straight line through the elements close to the radiator passes closer to the director at the other end than 0.25 wl" (These are "old" rules so maybe as much as 1dB was considered ok, but I think the loss is smaller)

I hope someone else will give simulation results with real numbers.

On 25-Sep-2001 G8MBI wrote:
>Has anybody measured or modelled the 
>possible effect in antenna gain due to boom sag?

yes.....

the result is complex depending on where the sag occurs, if it is uniform and which type of antenna..but within sensible limits was not severe....the exact results for those antennas I modelled are in an old modelling notebook not to hand, so rather than go into that.....

I would suggest using 'dacron cord' for the support ropes.

On 25-Sep-2001 K2TXB wrote:
Dacron is better than rope, but not the best answer. Dacron stretches a lot when new and has to be prestretched for about 2 days before making the final adjustment and putting up the antenna. And even then the Dacron cord on my 52 foot 2 meter antenna broke in about 1.5 years. I replaced the supports with phillistran (sp?). The improvement is dramatic. No stretching at all, and no breakage problems either. Simply adjust the turnbuckles until the antenna is straight and forget it.

On 25-Sep-2001 K6LEW wrote:
Ramiro, Russ makes a good point about Phillystran. We use this for truss support on all our long boom yagis now, the most recent being our new 19 element 2M yagi. We found this solution to work very well and recommend it.

On 25-Sep-2001 W3EME wrote:
I would suggest the antenna manufactures take a note from Cushcraft & use strut supports of aluminum....(ie; 17b2)

On 25-Sep-2001 K2TXB wrote:
Hi Brian. Aluminum is (IMHO) the worst solution of all. Aluminum struts fatigue and fail. I can tell you that I've seen many Cushcraft antennas fall apart on the tower. In all cases the aluminum struts are the first part to fail. Cushcraft antenna failure is not all due to aluminum strut failure however. CC seems to believe that the way to build a sturdy antenna is to throw size and weight into the components. But this just makes the parts even more susceptible to fatigue failures. I see the elements fall off, and the booms break. KLM, on the other hand, went to a strong but flexible boom supported by Phillystran. And M2 has followed suit in their designs. I've never had a failure in any of those antennas including one pair of KLM 16lbx's that were in the air continuously here for about ten years, and are still in good mechanical shape, although not in use at this time.

Unfortunately M2 has decided to cheapen their antennas by providing Dacron line for boom supports instead of Phillystran. I think that is a mistake, but it is not too hard to substitute Phillystran - in fact I bought the Phillystran from M2.


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