HELP: How to get lower VSWR??

[EdmundZheng]

vswr limites

Hi all!

I am working on a modified LPDA(Log-periodic-dipole atenna), and got good pattern, however, VSWR is bad in low band. I tried kinds of method to improve it, such as taper impedance matching, adding inductor, unfortunately, get bad results.
From smith chart, i found that in low band, the antenna had very large reactance, but adding inductor didnot work. So, how can i get good vswr(less than 2.5) in broad band? Thx in advance!!!


Regards!
Edmund

 

[jian]

Hi, Edmund:

I don't think you can do much when the frequency is below some limit. This limit is determined by the size of your antenna. If your antenna is obviously below quarter a wavelength at the frequency, you should not be able to get good match. You should get some high capacitive part in the impedance. Supposely, you can use an inductor to balance it. However, the resistance will still be small while the inductor will take the space too. In some sense, you can't save space. If you are not limited by space, you can make your antenna bigger to push the low frequency limit down anyway. Regards.

 

[raeza]

please try these parameters: tau=0.84, sigma=0.054
these parameters are optimized for a lower VSWR and minimum ant. lengths

 

[pit]

I am working on a modified LPDA(Log-periodic-dipole atenna), and got good pattern, however, VSWR is bad in low band.

What means "modified"?

Assuming that your dipole elements are long enough for the lowest frequency I got good results with my antenna designs when I made the longest dipole for a 7% lower frequency than required. For example: if the lowest required frequency is 80 MHz, I designed the longest dipole for 75 MHz. That resulted in a pretty good and flat VSWR at 80 MHz.

The next important issue is the coupling of the koax cable to the two booms, I got good results with a short piece of chopper formed in a double S, one end as wide as the boom itself, the other end as wide as the drilled-together cable. This avoids instant impedance changes. This is quite important and may spoil overall performance.

The third important thing is that the booms increase in distance to the lower frequency dipole elements. Look at the LPDAs from Schwarzbeck, their antennas are made in this way. This leads to a constant impedance. I started with the upper frequency limit, adjusted the boom distance for lowest VSWR, then I adjusted the lower end for minimum VSWR.

Another important issue is to balance the antenna with a ferrite bead of proper size (and in the case of transmission with proper power handling capability). Omitting this may result in distorted antenna pattern as there may be standing waves on the screen of of the cable.

With a proper design there is no requirement for external impedance matching, the LPDA can matched pretty good to 50 Ohm over the complete frequency span.


CU

 

[Azulykit]

If you really want to improve s11 and you have a sense of humor about what it will cost in gain, a trick is to add a little loss the feed. -3 dB of attenuation will improve s11 by -6 dB.

Pit's comments above is the best path to follow, namely, design for match and gain etc. This comment is a little flippant but it will help s11 at some expense to gain. The advantage is that it is quick and easy.

 

[Mohsinbinsohail]

What means "modified"?

Assuming that your dipole elements are long enough for the lowest frequency I got good results with my antenna designs when I made the longest dipole for a 7% lower frequency than required. For example: if the lowest required frequency is 80 MHz, I designed the longest dipole for 75 MHz. That resulted in a pretty good and flat VSWR at 80 MHz.

The next important issue is the coupling of the koax cable to the two booms, I got good results with a short piece of chopper formed in a double S, one end as wide as the boom itself, the other end as wide as the drilled-together cable. This avoids instant impedance changes. This is quite important and may spoil overall performance.

The third important thing is that the booms increase in distance to the lower frequency dipole elements. Look at the LPDAs from Schwarzbeck, their antennas are made in this way. This leads to a constant impedance. I started with the upper frequency limit, adjusted the boom distance for lowest VSWR, then I adjusted the lower end for minimum VSWR.

Another important issue is to balance the antenna with a ferrite bead of proper size (and in the case of transmission with proper power handling capability). Omitting this may result in distorted antenna pattern as there may be standing waves on the screen of of the cable.

With a proper design there is no requirement for external impedance matching, the LPDA can matched pretty good to 50 Ohm over the complete frequency span.


CU

We have been using tau=0.98 and we are getting VSWR of 44dB which is very high..... These calculations are from online calculator... We are exciting it at 800-900MHz...,We want LPDA for dual GSM band... How can i improve VSWR as i have to use this antenna in GSM signal strength enhancement system

 

[MANILKA]

Re: vswr limites

I think you need to think about having a BALUN transformer or similar balancing technique in order to obtain lower mismatch loss--thereby making the reflection coefficient tending to zero and hence vswr to 1. By making a small BALUN by a microstrip would solve your problem.

 

[Mohsinbinsohail]

Thanks for the valuable information...

I need a little more help...Can you please explain me how i can design and use Microstrip Balun in HFSS....I have a complete LPDA model prepared in HFSS but i dont know how to add or calculate the balun size? Please tell where to locate that balun? And kindly clear all the issues related to it.......Thankyou lot